Approved Rulebooks: Both editions of B/X, and the Rules Cyclopedia. Labyrinth Lord is close enough to B/X that it can be used as well, but don't use first edition or Swords & Wizardry, because they have a different ability modifier chart.
(Also: Ignore THAC0, AC, Hit Points, and weapon damages. These have all been standardized by new rules.)
Material from any version of AD&D may be used, if approved by the group in advance, but no mixed race/class combinations.
Character Classes: All character classes are grouped into three (or four) roles.
Three: Warrior (d8), Expert (d6), Adept (d4). Use that die for hit dice, and weapon damage. I stole the names from True20 'cause I didn't know what else to use.
Or we go with the not-so-Red-Box four-fold model (Warriors, Priests, Rogues, and Wizards, where Warriors get d10 hit dice, and Priests get d8 hit dice). The four-fold model suggests a world heavily dominated by militant religious orders, just by-the-by, which might be cool for a "more outre" setting.
Saving Throws: How 'bout we only use three? vs Death/Poison, vs Spells, and vs Breath Weapons. We could maybe use vs Paralysis/Petrification, if y'all want.
To Hit Bonus: Never mind THAC0. Your To Hit Bonus is equal to half your level rounded down, plus any other bonuses, like ability modifiers and weapon proficiencies.
Armour Class: We'll use the Ascending Armour Class. AAC is equal to 20 minus AC. This is the number you need to hit. So, normal people have AAC 11. I'll make up a new armour chart. You need armour proficiency skills to wear anything heavier than a chainmail shirt.
Hit Points: We'll use the Mule's abstract Hit Die system. You only roll them when you take damage, and when you get healed, they become unrolled.
Damage: Use whatever weapon you want. Your damage is your hit die. One or two sizes smaller if you're unarmed. Two-handed weapons get penalties in cramped conditions and won't let you use a shield, but they can get extra damage, extra speed, or a longer reach. Maybe there is a special thing where you can get two of those. I don't know what extra speed means, though.
Weapon Proficiencies: You get some weapon proficiencies based on role, each slot gives you a +1 bonus to hit with that weapon. Warriors get more weapon proficiencies as they advance through levels, which will account for their better ratings on the THAC0 chart.
Warriors start with 5 weapon proficiency slots. No one proficiency may be raised higher than your level +1. You may save your proficiency slots and fill them later. You get a new weapon proficiency every second level (2, 4, 6, 8, etc).
Experts start with 3 weapon proficiency slots. No one proficiency may be raised higher than your level. You may save your proficiency slots and fill them later. You get a new weapon proficiency every third level (3, 6, 9, etc).
Adepts start with 1 weapon proficiency slot. You may save it for later, in case you get that awesome magic weapon and you want to make sure you get as man bonuses to hit with it as possible. You get a new weapon proficiency every fourth level (4, 8, etc).
Skills: We use skills, too! You get three or four skills. You can roll one up on the random background chart in the second edition AD&D Player's Handbook if you like. If you have to roll for your skills, roll under Ability + Level. Or if you have to roll high, add your level and your ability modifier to the roll. You get a new skill every fourth level after level 1 (5, 9, 13, etc), or we'll speed that up and you get a new skill every third level (3, 6, 9, etc).
How to roll high: Roll d20, add modifiers, match or beat a target number (armour class, saving throw, somebody else's ability, etc).
How to roll low: Add modifiers to your ability, roll under on a d20. In some possible situations, roll under ability modified by penalties only, then add bonuses to the score. The higher the score, the better the effect. But rolling over is failure.
Changes to character classes:
Thieves: Thief skill are just special skills, no percentile ratings, which are dumb. Use them like any other skill, your bonus is your level. Same goes for any other class that has skill-like special abilities. Just write the skill names down on your sheet.
Elves: High Elves get magic user spells, and Wood Elves get druid spells (I haven't looked up how that works, but it sounds good). Elves get penalties for using cold iron implements, and have their own metals, like bronze and mithril.
Starting levels: We can start at 1st level, or we can start at 3rd level. If we start at 3rd, you can take more than one character class. You can spend your experience points training for any class that you can find an instructor for, or that you are already trained in. You have hit dice equal to your highest level. If another class gives you larger hit dice, use those instead of the lower hit dice. Or just start at 1st level and have one class.
Alignment: Law and Chaos are actual commitments to gods, as in Elric. We'll use Good, Evil, and Neutral as well, just as a rough personal guideline, and in case anybody ever takes the spell Detect Evil. If you want to be dedicated to the Chaos gods, you can roll for rewards and mutations on the charts in Realms of Chaos: Slaves to Darkness.
Experience Points: As per Rules Cyclopedia. You get lots of XP from treasure, fuck-all from defeating monsters and opponents, and 1/20th for roleplaying, achieving party goals, and performing exceptional deeds.
Optional — a nice reward: You get to distribute extra points between your abilities equal to your level. You don't have to spend the point(s) right away.
Armour and Ascending Armour Class: AAC starts at 11, add Dexterity modifier, and armour bonuses.
Shield +1, leather jacket +1, leather suit +2, studded leather suit +3, chainmail shirt +3, metal breastplate +4, anything heavier you need proficiency to wear or -2 penalty. Full chain suit +4, chain and plate, +6, full plate suit +8.