Recycling 5e subclass concepts into singular abilities to be found, learned, or defended against in a more old school game world. Consider this article. Also these ones. I’ve mostly left the mechanism of gaining the ability up to the DM, but the warlock subclasses are framed a bit more suggestively. Some abilities require the user to already possess a certain class, while many others won’t. Use common sense. Other abilities make available existing spell lists with the denotation "Learn [class] spells."
Potential uses of these abilities:
A faction of NPCs possess a subclass ability in common. They teach it to their members and allies, and employ it against their foes.
A manual discovered in a dungeon space shows the way to gain a subclass ability, requiring study and training.
An independent NPC mentor or patron can offer the ability as an arcane gift, but may demand favors for its continued use.
Using thankfully the compilation and formatting of u/Deathly_Drained.
Armorer - Shown the secrets of crafting +1 plate armor in a process requiring a week and 2,000 gold pieces. Mechanisms and magic items can be incorporated into the armor set. The secrets of more powerful enchantments are guarded by other arcane armorers.
Alchemist - Learn appropriate spells and brew them into potions as a magic-user of your level makes scrolls.
Artillerist - Fortification magic allows the conjuration of firearms, flame traps, and cannons.
Battle Smith - Learn the art of crafting mechanical soldiers with supplies equal in cost to three years’ wage of the equivalent mercenary.
Berserker - Before a battle, enter a frothing rage. Attack twice a round, but cannot forgo attacks if possible-- if you fell a foe and your ally is next to you, you might have to attack them with your second attack.
Totem Warrior - After completing a spiritual initiation, the DM picks out a monster from the manual and shows its stats. When convenient, you may act as though you had those stats. If your DM says you’re still not allowed to fly or something, then I’m overruling them and saying you can do it once, ever.
Ancestral Guardian - Cloud of your ghostly ancestors give nearby allies +1 AC. Will do up to three additional favors per day: can fly out to Cause Fear, locate objects (or creatures as Locate Object), or give their advice on a difficult situation.
Storm Herald- When in life or death situations, a storm erupts around you, obscuring vision and hearing and damaging those within ten feet (as insect swarm).
Zealot - Immune to magic cast against you by filthy heretics. Raise dead has no time limit for you and you leap from the grave immediately at full strength.
Beast - Develop a predator’s claws, tail, or biting maw. Swim like a fish, climb like a monkey, or jump like a cougar.
Wild Soul - When you roll a natural 1 to hit, roll on this post’s wild magic table.
Battlerager - When you wear spiked or serrated armor, you can make an extra attack each round for 1d4 damage. This attack may automatically hit if wrestling.
College of Lore - Learn spells of other classes’ spell list. May spend the normal cost of consulting sages in sending for books and conducting rituals in any moderately sized town to be your own sage.
College of Valor - Proficiency with chain armor and shields. On a hit, may expend a memorized spell to do an extra d6 damage.
College of Creation - Sing objects into being, summoning anything the size of a sword or larger at a rate of one minute per gold piece in cost. Others in the college know the secret of singing life into objects or conjuring vast constructions, but they guard their creative process jealously.
College of Glamor - Charm and Dominate spells may affect up to 4d6 HD of random rubes.
College of Swords - Proficiency with swords, and may reroll 1s for damage.
College of Whispers - Scare foes as though threatening them. When you witness someone die, bottle their shadow. Drinking a shadow confers their appearance, mannerisms, and knowledge of general background for one hour.
College of Eloquence - primp and groom with magic toiletries each day to get +1 reaction roll against humans. Will be burned at the stake if your magical talent is uncovered.
College of Spirits - Tell the tale of a random spirit, and they will control you for the next ten minutes, lending their skills and biases. Roll a d100 on this table.
Knowledge Domain - Spend a spell slot to know how to use an object.
Life Domain - Cast Cure Light Wounds an extra three times each day, seven on holy days.
Light Domain - Cast Light an extra time each day, and you can cast it in response to being attacked.
Nature Domain - Learn druid spells.
Tempest Domain - Learn Control Weather and cast it an extra time each day, even if it’s normally too high-level for you
Trickery Domain - Learn illusionist spells.
War Domain - When an ally misses a foe, you may retroactively give them +10 to hit as distant thunder gives off a perkodhuskurunbarggruauyagokgorlayorgromgremmitghundhurthrumathunaradidillifaititillibumullunukkunun. May use this ability again after rolling a 20 on an attack roll to hit a foe.
Death Domain - Learn necromancer spells
Twilight Domain - See in dim light. Your party regains 1 extra HP with every twilight.
Order Domain - Once per round, command a willing character to do something instead of otherwise acting on your turn and they may do it immediately.
Forge Domain - Identify magic metal objects, and spend an hour with a fire and a hammer to give a weapon or set of armor +1 for the day.
Grave Domain - A creature within 20 feet of you can’t fully die unless you will it or unless double-tapped
Peace Domain - +1 Reaction when totally unarmed. When you officiate a wedding, the newlyweds get +1 to all saves as long as the marriage is a happy one.
Arcane Domain - Learn magic-user spells
Roll a random artificer subclass.
Roll a random warlock subclass
Circle of the Land - Commune with the leylines of a certain terrain type each sunrise to get +1 spell per spell level you know.
Circle of the Moon - Bite an animal with HD equal to your level or less. If it dies in the next hour, you can turn into it, once, until you choose to transform back. Transforming is nearly instant, like that one Bilbo Baggins jump scare.
Circle of Dreams - No one can sneak up on you while you’re resting and you can take 1 wisdom damage to walk an occulted pathway to any point in 60 feet, as long as you can see your endpoint.
Circle of the Shepherd - Talk to animals. If an animal owes you a favor, you may summon them to aid you until they consider the favor discharged.
Circle of Spores - Give off a halo of spores, doing 1 damage to adjacent living foes at the end of your turn. If this kills someone, the spores infect them, allowing you to command them like a zombie.
Circle of Stars - Each night, consult the night sky and a star chart to gain a random magic-user spell of a level you can cast the following day.
Circle of Wildfire - Friendship of a lesser fire elemental.
Roll a random barbarian subclass
Champion - On an attack roll of 20, you have slain your foe. Giant monsters like dragons may have enough time to mutter curses.
Battle Master - See a foe’s HD by watching them fight. +2 to aggressive maneuvers that aren’t attacks.
Eldritch Knight - Cast magic-user spells at the speed a cleric learns spells.
Arcane Archer - Learn spells and fletch them into arrows like a magic-user scribes scrolls.
Cavalier - Never fall off your horse unless you want to. If you do, you do a cool roll and hit the ground running.
Samurai - Roll attacks with advantage when wielding a one-handed weapon. Roll an attack for each weapon if wielding two. If slain, you can remain standing for up to a minute.
Psi Warrior - Cast ESP and Telekinesis at will. Certain monster will want to eat your brain.
Rune Knight - Learn Giant language and inscribe your personal rune on your weapon for +1 to hit and one other tool for +2 or +1-in-6 to whatever it does. Grow taller and stronger than a normal human, getting +3 to strength.
Echo Fighter - Summon a ghostly duplicate with 1 HP that never goes more than 30 feet from you. Each round, you can swap places with it, and you can make your attacks originate from it. Treat it as an allied combatant, if that becomes relevant. Sometimes shows signs of sardonic independent thought. If destroyed, you can pull a new one out of any reflective surface.
Purple Dragon Knight - When you charge into battle, you and anyone charging with you gain 1d4 HP if you give a battle cry. If you go over your maximum HP, the excess sticks around for the battle.
Way of the Open Hand - Unarmed attacks deal d4 damage. On a 1 to damage humanoids, they must save or be paralyzed for 1d4 rounds.
Way of the Shadow - Your shadow puppets are totally convincing. Can Move Shadowly, flowing from one shadow to another with supernatural facility. Chance of success is equal to a thief’s Move Silently chance, and on a failure you are detected and waylaid.
Way of the Four Elements - Unarmed strikes imbued with 1d4 of the four elements to deal d6 damage, and knock a foe five feet back.
Way of Mercy - You can punch 1d4 of your own HP into an ally, even reviving them if their brain hasn’t died yet. Your palm strikes inflict 1d6 rot damage.
Way of the Astral Self - Send your spirit out of your body to walk around while your body stays stationary. The spirit can still take damage like normal, and if slain you are both brain-dead and soulless, but so projecting you are translucent, may deliver a flurry of strikes to every foe within 10 feet each round for 1d4 damage, and use your wisdom modifier to attack and damage rolls instead of strength.
Way of the Drunken Master - When drunk, no one will believe you’re intentionally attacking them and will consider you the least prominent combatant. No penalty for fighting retreats.
Way of the Kensei - Proficiency with all cool weapons. If you miss an attack, may deal 1d3 unarmed damage anyway
Way of the Sun Soul - Kicks have the range and panache of a flaming javelin. Can burn ghosts.
Way of Long Death - Unarmed attacks deal 1d6 damage, and if they kill a foe you regain HP equal to their HD, rounded down. When you give a glowering smile, people suddenly realize the depth of your depravity. You are seeking the enlightenment that kills death, and you’ve met masters who have found it.
Way of the Ascendant Dragon - Punch and kick fire or poison or lizards or something up to 30 feet for 1d6 damage. Dragons of your tradition won’t attack as long as you’re respectful. Your master’s master claims they got their scaly wings through enlightenment.
Oath of Devotion - As long as you uphold your oath, your weapons glow in the presence of demons and the undead, and you deal an extra 1d8 damage against them.
Oath of the Ancients - As long as you uphold your oath, you and those behind you take half damage from spells and get +1 reaction with animals. You are not slowed by poor weather.
Oath of Vengeance - As long as you uphold your oath, you deal double damage against someone who almost killed you, killed another party member, or otherwise deeply wronged you. Keep avenging long enough, and the avenging angel in your dreams promises you will be as her.
Oath of Conquest - As long as you uphold your oath, if enemies would flee you, they instead cower and take damage equal to your level.
Oath of Redemption - - As long as you uphold your oath, if those you defeat swear to forswear violence or specific evils, they are bound by a Geas to do so. Your Detect Evil spell expresses evil as a percentage from 1 to 90.
Oath of Glory - As long as you uphold your oath, your fighting style is dazzling and beautiful, and if an adjacent foe would be hit by an attack roll of 20, you can make a quick attack first to try to steal the credit.
Oath of the Watchers - As long as you uphold your oath, you get the benefits of a Protection From Evil spell constantly.
Oath of the Crown - As long as you uphold your oath, if you challenge someone nearby to a duel they can’t move more than 30 feet away from you until you attack someone else, one of you is defeated, or something.
Oathbreaker - Since you broke your oath, you’re evil and goth now. Villainous NPCs and mindless undead will respect your work. Deal +1d6 damage against former friends and mentors.
Roll a random monk subclass and retain it as long as you uphold your oath
Fey Wanderer - Grow antlers. Cast Charm Person and Dispel Magic once a week.
Swarmkeeper - Carry a horrific swarm on your person. It attacks those who wish you harm.
Gloom Stalker - See in the dark and deal double damage in the first round of a fray
Horizon Walker - Instantly acclimate to new dimensions of existence. Once each minute, you can disappear and reappear within 90 feet rather than bother to walk the distance.
Monster Slayer - +1 damage
Hunter - +1 to hit
Beast Master - as Carcass Crawler Beast Master class
Drakewarden - A dragon spirit haunts you, and you can bring it to life with a magic invocation. If it dies, you can call it back from death on the day after the sabbath.
Thief - +20% Move Silently and Pick Pockets
Assassin - as OSE assassin class
Arcane Trickster - first level spell slots as a magic-user, can only learn Mage Hand, a spell that lets you make fine manipulations from up to 20 feet away.
Inquisitive - +20% to find/remove trap, and can use the skill to notice discrepancies in a story
Phantom - Use Hear Noise skill to ask the dead in an area one question.
Mastermind - After speaking to a creature for a minute, you can tell the number of HD and special abilities it possesses
Scout - Can use Climb Sheer Surfaces to track across difficult terrain, and may get the Backstab bonus for attacking unaware foes with a bow or other ranged weapon.
Soulknife - Summon a colorful psychic sword at will. Use Pick Pockets skill to read surface thoughts, detected on a failure. Communicate telepathically with anyone who shares a language within 100 feet.
Swashbuckler - Proficiency with shields. No penalty for fighting retreat. If killed, have time to utter romantic poetry first.
Roll a random ranger subclass
Aberrant Mind - Three times per day, may cast Calm Emotions or ESP. The alien beings that did this to you will want to eat your brain.
Clockwork Soul - Your organs faintly tick. You roll 2d10 instead of d20s.
Divine Soul - Learn cleric spells.
Shadow Magic - See 20 feet in the dark. Spend a second-level spell to summon a black wolf.
Storm Sorcery - In addition to normal movement, fly on gusts up to 10 feet each round.
Draconic Bloodline - Take half damage from your ancestor’s element, and spells of that kind you cast deal +2 damage.
Lunar Magic - When the moon’s waxing, cast Faerie Fire once a day. When the moon’s waning, cast Darkness once a day. During the new moon and full moon, get both.
Wild Magic - 10% chance per spell that you get to roll on this post’s wild magic table.
The Archfey - You sold something important to a fairy queen, and now positive reaction rolls are charmed by you and negative reaction rolls are at least a little frightened by you
The Fiend - You sold your soul to a marquis of Hell, and now fires you start spread as quickly as possible and you’re mostly immune (DR 5) to the fires of Earth.
The Great Old One - You agreed to something you don’t understand, and now you’re immune to the negative effects of further insanities and curses, and they’re contagious as long as you carry them.
The Celestial - You pledged to serve the good, and now you can learn cleric spells and perform baptisms
Undying - You were desperate to juke life, and now you can cast non-animating necromancy and curse spells. If you would die, make a save vs death the next time someone approaches your body to yop triumphantly and grab their ankles, reset to 1 HP.
The Hexblade - You took up a curséd blade. See the Goblin Punch demon blade class.
The Fathomless - Under duress, you sold your service to Davy Jones, and now you can swim like a fish, sail like Francis Drake, and summon a man-sized octopus when on the ocean.
The Genie - You freed a genie from a mundane vessel, and now can enter its extradimensional living space. If you ever find that genie again, they said they owed you a wish.
The Undead - You were desperate to juke death, and now you can learn necromancer spells and don’t need to breathe.
Roll a random cleric subclass. You sold your soul to gain its powers.
School of Abjuration - When you cast a spell, you weave the residual ectoplasm in a magic shield that gives you DR equal to the spell level for a round. If someone would disrupt a spell you’re casting, there’s a 50% chance they’re thrown back and take 1d4 damage instead.
School of Conjuration - Summon small, simple items from your sleeve. Thrown a cloak or sheet over your head to teleport up to 30 feet away where no one happened to be looking.
School of Divination - Perform divining ritual each day and roll 2d20. If someone would roll a d20 that day, you can declare they get one of the two d20 rolls you made instead.
School of Enchantment - When maintaining intense eye contact, the targets of your spells get a penalty to their saves equal to your level.
School of Evocation - Your magical blasts, bursts, rays, and the like skim around your allies. Beware, you are never immune to your own blasts. May permanently spend a tenth of the blood in your body (lose 1 CON) to deal maximum damage with a spell.
School of Illusion - Illusions you cast can be made real, but never as long as you wish
School of Necromancy - Learn necromancer spells, and when your magic kills a creature you can huff its life force to gain HP equal to its HD, rounded down.
School of Transmutation - Wave a wand over an item to transmute it from one substance to another that a medieval peasant would be familiar with. Lasts an hour unless you use the spell again. There are transmuters who know how to do these changes permanently, but they’re too rich and powerful to teach you. You hear they’re young, immortal, and hot 😔.
School of Graviturgy - May set your density between 1% and 200%. Every hour on the hour, may set your personal center of gravity. It’s said the ancients knew how to set centers of gravity for other creatures and objects.
School of Chronurgy - Immune to Time Stop spells, adding about an extra hour to your workday. When you cast a spell you can encase it in a bead of Time for up to an hour. Damaging the bead causes the spell to go off. One per standard Chrono-Unit (28 hours), you can summon a random clone from another timeline. This is dangerous, because they are never exactly what you expect.
War Magic - Focusing, you can charge magical energies, taking one round per damage die of the spell you’re charging. Once charged, you can release the spell to deal maximum damage. While charging, you can’t cast spells, attack, or run, ex chetera. If damaged while charging, you must save vs spells or explode for 2d6 damage to yourself and everyone within 10 feet of you.
Bladesinging - When sword dancing, you know how to use swords and your robes count as light armor.
Order of Scribes - Your quills never run out of ink, halving the time it takes to copy scrolls and books, and to learn spells. NPC magic-users will get incredibly jealous. Quillwork is tedious.
Roll a random artificer subclass
Roll a random sorcerer subclass, attained with potions and experimentation
Roll a random druid subclass, attained with biomancy