Assuming we’re starting from the original three booklets as the base, I now make these changes before adding any supplemental material (from Greyhawk, for example.)
- Corrected d6-only hit dice progression. Fighters get 1d6 per level up to 10th. Magic-Users get 1d6 every other level. Clerics are in between (2 dice every 3 levels.) M-Us and Clerics get a “loaner” hit point on levels where they don’t get a hit die. “To Hit” chances are based on hit dice, sort of like a loose interpretation of Target 20.
- Situation Rolls. All the various probabilities for surprise, listening at doors, opening stuck doors, wandering monster rolls, and so on are regularized to 5+ on 1d6. Anytime I decide something could go wrong with an action or players have a slight chance of escaping a bad situation, I pretty much use that roll.
- Magic Scrolls. Only magic-users can make magic scrolls of any type, including cleric scrolls. Magic-users can use any scroll type, Clerics can use cleric magic scrolls, and anyone can use protection scrolls or curses. Magic-users can make scrolls at any level, as per Holmes Basic, but only of spells they know or research. Spells above 6th level can’t be memorized, only cast from scrolls.
- Clerics Without Spells. Clerics don’t memorize spells, but use the Turn Undead roll to pray for miracles. Use the reaction roll table as a replacement, 2d6 + 2 * (cleric level - spell level or undead hit dice). 9+ means spell is cast or undead is turned. The first time a cleric gets a 6-8 result, the spell asked for is cast, but never again for the rest of the adventure. On a 2, the cleric falls from grace and loses all powers. Praying at an appropriate shrine and otherwise fulfilling religious duties erases all penalties and starts over if a 9+ on the reaction roll is rolled. (The link is to the first post of many about using Clerics Without Spells, but these rules represent my current thinking.)
- Corrected Armor/Shields. Leather armor has no move penalty for fighters, -3 Move for everyone else. All metal armor halves base Move. Magic-Users (and some custom classes) halve Move again and must rest 2 turns out of every 6 when wearing metal armor because of fatigue. (I use this general fatigue rule elsewhere, too.) Shields shall be splintered, but only for fighters.
There are certainly other, lesser fixes, some of which are still in the midst of being tweaked. Some things I also add are background professions, special abilities of weapons that take effect when damage is 5+ or when a critical hit is rolled, changes to Read Magic and the way spells are learned.
Most of the changes to damage, combat, and ability scores from Greyhawk are ignored. Thieves are still being tweaked, but they get the same hit dice as Magic-Users. A lot of custom classes wind up being Thieves or Clerics with replacement abilities. New spells from the supplements might require reworking before I use them, but new monsters and magic items are generally OK.

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