All right, here's a general outline of what I have in mind. I'll write all this up as a proper ruleset later, with all the little explanations like "What are ability scores?" and "What's a DC?" For now, I'm concentrating on how the rules work and what the thinking behind them is.
THE BASICS
Ability Scores
Your character has the usual six ability scores. You can generate them using any of the standard methods; however, the recommended method is:
- Start with 12 in each score.
- Roll 1d6. Add the result to one score of your choice, and subtract it from another. You cannot add to a score if it would raise that score over 18, or subtract if it would reduce it below 3.
- Repeat 3 more times. (Do not roll all four dice at once; you decide where to put each die before knowing what the next roll will be.)
This system preserves the idea of "rolling for stats," producing some unpredictable results, while giving players a fair bit of control over the outcome and overall balanced numbers. It's also easy to understand and quick.
Ability scores don't have any direct impact on the core combat mechanics. You don't deal more damage for having a high Strength, or get more hit points for a high Constitution; those are functions of your class and level. The main use of ability scores is to make
ability checks, which are simple: Roll 1d20 and add your ability score (not a calculated modifier, the whole thing). If you're making an opposed check, the DC is equal to 10 plus your opponent's ability score. Otherwise, the DC depends on what you're doing, with 20 being the standard.
Ability checks are used for:
- Resolving special combat effects. For instance, if you're poisoned, you make a Constitution check to resist. (As a general principle, you have to hit with an attack or spell before you can use an ability check against an enemy. This is to prevent low-level characters from exploiting ability check-based tactics to take down high-level foes, since ability scores don't scale by level.)
- Resolving "stunts." Typically, when you want to do a stunt-type move, you make an ability check at the start of your turn. If you succeed, you get to do the stunt. If you fail, you mess up and lose your turn. This is obviously subject to DM judgement and the stunt being attempted.
- Most noncombat activities. Stealth is a Dexterity check versus Wisdom; swimming a river is a Strength check; convincing the dragon not to eat you is a Charisma check; et cetera. There will be defined rules for a handful of these, while the rest will be left to DM judgement.
Combat Basics
Characters have two defenses: Armor Class (AC) and Magic Resistance (MR). All physical attacks target AC, and all supernatural or magical attacks target MR. Both are determined by class and level. Your AC is also affected by your armor and shield, if any.
Attacks are made using a standard attack bonus; again, this is class- and level-based. When casting a spell, you make a spellcasting roll versus MR using your "spell bonus," which is analogous to attack bonus. Pretty simple stuff. The rate of scaling for both attacks and defenses is 1 per 3 levels up to level 12 ("name level"), then 1 per 6 levels thereafter. This is deliberately slow, so that you can have workable combats involving foes at a wide range of levels. A 20th-level character should be able to have a fun and dangerous combat against orcs... it just takes a whole lot of orcs!
When someone does hit you, you have
life points and
hit points (wound points/vit points, if you prefer). Hit points increase by level, are lost first when you are injured, and refresh with a 5-minute rest. Life points increase only very slowly, are lost only after you run out of hit points, and can only heal with clerical rituals or days of rest. If you are reduced to zero or less life, you fall unconscious until your life is positive again. Each round, you make a Constitution check at DC 20. Three failed checks and you die; roll a natural 20 and you stabilize but remain unconscious. Another PC can stabilize you with a standard action. If your life is reduced to negative (10 plus your level), you die outright.
Unlike some wound/vit systems, this one does not allow crits--or much of anything else--to bypass hit points. As long as you are conscious and able to move, your hit points form a near-impenetrable shield around your life. After all, the whole point of an ablative hit point mechanic is to prevent sudden arbitrary death! In the very rare case that something does bypass hit points (a wraith's attack, maybe), it will do so consistently.
Initiative and actions follow the 3E model for now: Fixed initiative cycle (initiative = Dexterity check), standard and move action each round. I'm trying to avoid adding minor actions. For purposes of this game, any mechanic that requires minor action is probably too complicated.

I might experiment with a more old-school initiative system, where you roll each round and declare before acting, if it seems like it wouldn't bog down the game.
Races
Okay, I'll admit it: I don't like the standard races. I think they're a bunch of cardboard Tolkien rejects who get crammed into every damn D&D setting regardless of whether they contribute anything. So my first inclination is to say, "Screw it, humans only." However, elves and dwarves and halflings have a long and storied history in D&D, and it doesn't seem quite fair to just exterminate them. What do other people think? Are nonhumans sufficiently central to BECMI that they need to be included?
Next up: The fighter.