I'm fond of saying that that each campaign should have its own ruleset, even if it's just a couple house rules you tack onto your favorite retroclone or GLoGhack. I also like to say that it's good when the ruleset, setting, and campaign format are enmeshed together. To try to give a practical example, I here attempt to record my thinking for a game I hope to run in the future.
Brother GLoG
Talking to my brother, we decided that it would be neat if we could play some kind of casual tabletop roleplaying game together. It's been a while since it was just us, and I like running dungeoncrawls but don't usually get the chance, while he is interested in playing dungeoncrawls but doesn't usually get the chance. However, there are big constraints on running a duet game any time in the next few years. We're both busy people, and between his job and parenting he can't reliably set aside time to come over and play. So any game we play would be best served by being something I could bring over to his place and run in desperate snatches between child-related interruptions, or while walking around the neighborhood.
The physical constraints of being something that I can carry around are big ones. I'm thinking of trying to fit most of my notes into my phone and sticking any maps or character sheets in a binder. For the dice, I'm imagining one of those hard, transparent plastic deck boxes, sealing inside a d20 and four differently-colored d6s. That way, I can just shake the box and look down it.
I don't want to go towards an ultralight ruleset for this project. I want to track things like dungeon Turns, torch durations, inventory, and other classic dungeoncrawling concerns. That's where a lot of the dungeoncrawling game is for me. For the ease of cultural touchstones, I think I'll focus my worldbuilding concept on a fantastic reimagination of Rennaisance Italy, one of my brother's most beloved historical periods. We can semirandomly generate more of the region in a country-as-dungeon format as we find the need. At first, I'll just put the PC(s) in an area near to the Incandescent Grottos (but Italian), Little Snake's Man's Cave, and the Meal of Oshregaal(io).
In theory, I could use some kind of B/X chassis, but it's been a while since I've run GLoG, and its class design, which features discrete and weighty abilities, appeals to me. (I may soon write a manifesto on the importance of weighty choices.)
Following with this goal, and hoping to eschew extra numbers, I plan to follow in the footsteps of Numbers Aren't Real's G20 and have primary attributes simply be ability modifiers, with no scores numbering 3-18. I'll also cut down the primary attributes into four scores for the same reason:
Fate: to spell saves
Luck: AC, getting picked on by the DM, XP modifiers
Arete: to hit and damage, languages known
Bravery: to HP, reaction and loyalty rolls, breaking down doors
To make a character, you roll a d6 for each primary attribute (which I will call ideals from this point on.) On a 1, it's -2. On a 2, it's a -1. On a 3 or 4, it's +0. On a 5, it's +1. On a 6, it's +2. Then, roll a d20 to determine your upbringing:
1-7 Subject: +1 Fate. Cannot ride horses or read
8-13 Citizen: +1 Luck. Endless rivalries.
14-16 Foreign: +1 Arete. No legal standing.
17-20 Noble: +1 Bravery. Endless obligations.
You start with 6 HP, 10 AC, equipment based on class, and a number of gold florins equal to what you rolled for your upbringing. All of this, including reaction rolls and saves and so on, are old hat for me and very little cognitive load.
Initiative: each round, the player declares what he wants his party to do, and the DM adjudicates it. Generally, ranged attacks and pikes strike first and spells go off last. Normal weapons deal 1d6 damage.
Combat: Actually, since I want to cut down on unneccessary rolling, screw AC, maybe I should try out oddomatic combat. Maybe all attacks hit outside of extreme circumstances, and you ignore damage rolls equal to your luck bonus or bonus from your armor. Let's go with that. This means that if there are 4 PCs and they all attack, my brother can just shake the plastic dice box once and get all four attacks
Armor: A shield negates damage rolls of 3. Fabric armor negates damage rolls of 4. Chain armor negates damage rolls of 5. Plate armor negates damage rolls of 6. Sometimes monsters or freaks will roll multiple damage dice. Armor negates all matching dice rolled.
Inventory: I kind of want to just say each PC gets 5 "equipped" slots and 15 "stored" stots. That's a lot of items for one person to track, so we will have to see if it works out.
Saves: You add the relevant ideal to a d20 roll, typically vs. 13.
Level up: at the rate of a BX fighter. Get +1 to saves and +1d6+Bravery HP.
Languages: I'll make my brother name and describe nearby countries as they come up, and say what monsters are famous for originating there. Thus a language is born. Quantum languages will be in play.
Format: as a frequently-interrupted dungeoncrawl, I don't want to plan too much for long downtimes except what can be done over discord between sessions. This is why I think I can probably manage something as complicated as B/X-style XP distribution can be-- it won't be reckoned during the face-to-face interaction. The classes will be designed with ease of memory in mind, and won't have a lot of dice pool mechanics. For simplicity, the only extra resource to track should be spells. I'll precommit that the mage class(es) will just have a number of spells per day, and magic dice will be absent. As cool as they are, and as central to GLoG, we won't remember how many dice were spent after weeks between sessions. We're more likely to be able to just tally spells as they are cast.
Starting Character Types
Magic weapons will only bear a mercenary, knight, slayer, augur, or angel to wield them, unless specifically for another type or something. No armor for mages or augurs. Dogs need armor that suits them. Multiclass freely, but you can only have four templates unless you do something crazy.
Try to infer why I went with some abilities and not others.
Mercenary (start with colorful leather armor, three torches, and a sword OR a pike and knife.)
A: +1 damage with weapons. You can treat your Fate as your Luck for saves, and vice-versa.
B: Appraise item value on sight. May break your weapon or armor to negate a d6 of damage.
C: When you lay low a foe, you may use an unused d6 in the dice box to attack someone else.
D: Determine an ethos. Your hirelings and retainers fall back on this ethos if they fail morale saves, and you can make a Luck or Fate save to get a hint, a feeling of certainty, along the lines of that ethos.
Knight (start with chain armor, a shield [with your arms], a lance, a charger, a misericorde, and a code)
A: +1 damage with melee weapons. When you charge or run someone down, roll an extra d6 of damage.
B: Even monstrous foes will try to treat with you, take you hostage rather than kill you, etc.
C: Your lance deals 4d6 damage on the charge.
D: You may take attacks meant for others nearby after seeing the results of the dice.
Slayer (start with a knife, a knife, a dagger, a swordbreaker, a theatre mask, a hood, and rope)
A: When you stab someone in the back, they make a Luck save or die.
B: When you wear a ceramic or wooden mask, no one can tell who you are. Climb silently and capably.
C: Climb at running speed. When you stab a monster in a vulnerable area, it must make a Fate save or suffer a dismemberment.
D: When you make a creature's face into a mask, it serves as a good disguise. If they weren't a humanoid you may need to add prostheses to be convincing.
Mage (start with a polearm, a signet ring, a crowbar, a lamp, and a mirror)
A: Cast 1 spell per day.
B: Cast 3 spells per day. Make scrolls.
C: Cast 6 spells per day.
D: Cast 10 spells per day.
Augur (start with three torches, a sharp knife, a drill, fabric regalia armor, and a holy symbol)
A: You can interpret horoscopes, predict the weather, assuage the undead (opposed 2d6 Fate saves), and slaughter kine for omens.
B: Cast 1 spell per day.
C: Cast 3 spells per day. Make curse tablets.
D: Cast 6 spells per day. Meditate for prophecies.
Angel (start with a charred scimitar, a flower, wine, and a harp)
A: Speak with plants and ark-descended animals. +1 damage with weapons.
B: Cast 1 spell per day. Immune to paralysis. Eyes like searchlights in the darkness.
C: Cast 3 spells per day. You may attack a foe in melee twice, but they simultaneously get to attack you once.
D: Cast 6 spells per day. By meditating, you open your first eye and see through hidden corners, revealing hidden doors and secret passages.
Genius (start with a book on three topics of your choice, a telescope, a crossbow, bolts, hammer and chisel, a vial of acid, and a shield)
A: Detect construction tricks and understand art by scrutinizing an area. Sense your way in the dark.
B: Can direct labor to take half the time. Reduce the chance that the party is surprised by 1-in-6.
C: In combat, coach an ally to give them +1d6 damage.
D: Construct war machines of incredible potential. Discern between a room that is empty and one where the inhabitants are silent.
Dog (start with a collar. You're a special canine humanoid creature with clumsy hands and speech, like a mole in Redwall.)
A: Your bite deals damage like a weapon. You have impeccable smell out to 60'.
B: Detect air currents. Dig at crawling speed.
C: May enter a barking frenzy. Foes get -1 morale and you get +1 to all ideals. On a damage roll of 6, knock a target prone. If you get a 6 on a prone target, tear their throat out-- they must make an Arete save or die.
D: Track by smell at an arbitrary distance. Attain an instant canine spy network in any city or town you visit.