AD&D is not D&D. That might sound trite and/or stupid, but it's true. A D&D character (B/X, Mentzer, Labyrinth Lord) with "low" stats, say all 9s, is extremely viable. The same character in AD&D barely qualifies for any class, has no bonuses of any sort and can't cast spells with any sort of proficiency. Oddly enough, anything over 14 is generally required to get a bonus. Hence high stats in AD&D != high stats in D&D. This is simply justification for the "high powered" stat rolling system I'm going to use. Yep.
1) Roll For Stats: 6D6, 5D6, 5D6, 4D6, 4D6, 4D6, 3D6, 3D6, 3D6
Keep the best six rolls, arrange as desired. This should result in at least three stats getting bonuses (if my math is correct), and qualification for nearly all classes.
2) Gnomes receive a +1 INT, -1 WIS (this is right out of 2nd edition and I like it)
3) Half-orc fighters who roll an 18 for STR receive an 18/00 w/their +1. This is over the racial maximum. So what.
4) No demi-human level limits. None!
5) Humans receive a secret bonus...
6) No weapon proficiencies; characters are proficient in all allowed weapons per their class.
7) Fighters (no rangers) may specialize per Unearthed Arcana, only at 1st level. They get to pick one weapon for the bonus. At 7th level they are considered "double specialized".
8) Available classes: Fighter, Cleric, Thief, Magic-User, Paladin (PHB version), Ranger, Barbarian, Assassin (may be neutral), Druid, Illusionist, Monk, Bard (both 1st and 2nd edition versions). 2nd edition Bards can cast Druid, MU AND Illusionist spells, but as these must all be learned (no automatic gain) I don't think this will be overpowering.
9) Pure Magic-Users (not multi-classed) can Detect Magic at will, just like the spell. As they gain levels, this ability will become more refined. Somewhere around 10th - 12th level, they will essentially be able to Identify magic items without a spell.
10) Initiative, yeah...2nd edition has the right idea, so here's how it'll work: D10 rolled every round for every character. Modified by weapon speed(!), spell casting time (segments), DEX. Multiple attack routines (high level fighters) will go first/last, regardless of initiative (per 1st edition). All actions must be declared at the beginning of the round. Spell interruption should be a lot easier to figure out. All magic items (wands, rods, whatever) will have a "weapon speed" of 0, with no interruption possible. This should make such items much more appealing.
11) For character levels 1-3, experience points for treasure and monsters is gained at 10X the normal rate. E.g., 100gp = 1000XP. This will allow accelerated level gain without a glut of treasure. After level 3, normal XP will be gained. In the case of multi-class characters with levels that fall into both categories (2/3 Fighter/Thief for instance), XP will be divided normally, then applied using the above rules, i.e. 100XP = 500XP for Fighter, 50 XP for Thief.
12) Clerics must pick a god from Deities and Demigods (or wherever) and may use the preferred weapon of their god without penalty. Clerics who do not proselytize properly will lose their abilities as I see fit.
Also this. I'm going to "be a rebel" and not answer. Not yet, at least.