Remathilis
Legend
I left out "as a jumping off point for something like AD&D". I want the options of AD&D without the cruft. So starting from RC and adding complexity (cruft I like) might work better than starting from AD&D and decrufting it.
BF lacks the added cruft I like. Arguably BF is all you need, I know. But I'd like some of the "advanced" bits thrown in as well. But, that's a different thread.
Well, since you put it THAT way...
Start with BF.
Add Gnome, Half-Elf, Half-Orc and Half-Ogre races. Allow a bit more Multi-classing options than F/Mu (I'd like F/Th, Mu/Thf as well).
Add a Ranger, Paladin, Druid, Illusionist, Bard, Monk, and Assassin class.
Add 7th-9th level Mu Spells, 7th level Cleric Spells. Fluff out both lists.
Allow Clerics spells at first level.
Give magic-users a bonus spell per plus of Int (13-15 1st level, 16-17 2nd, 18 3rd). Give thieves bonus to PP, OL, RT, MS, and Hd based on Dex plus (+1=5%).
Give races bonuses to thief skills. Allow Backstab multiplier to increase with level.
Add a simple skill system to the game, something a cross between secondary skills and non-weapon proficiencies.
Allow Fighters something akin to weapon-specialization.
Expand the Monster's Section to include a LOT more monsters, including more complex ones like liches, demons, and such.
Give monsters some additional info (Int, Alignment)
Add a lot of magic-items, I actually would borrow 3e's "plused" system (flaming = +1, etc) but not assign costs to items.
Add some additional situational bonuses (higher ground, flanking) to combat, allow some simple fighter combat options (dual-wielding, parry, smash, disarm, grapple, trip, etc).
Add back the 9 alignments.
You can go further if you want (adding back 1e racial traits, for example) but honestly I could run for YEARS off something this simple.