If half of the stuff in this thread came about, I'd have to take out a second mortgage on my soul to afford it all!
I'd love to see;
Greyhawk CS
Al-Qadim CS
Kara-Tur CS
Spelljammer CS
Mystara CS
Planescape CS
[All single-book hardcovers, like the FRCS or ECS, leaving future development dependent on sales. Don't flood the market if the stuff doesn't sell!]
Complete Giants
Complete Fey
Complete Constructs
Complete Goblinoids
Woodlands region book (a la Stormwrack, Frostburn, Sandstorm)
Swamps region book (ditto)
Arms & Equipment 2
Alchemy & Herbalism
Complete Skills (with heavy focus on use of Crafting skills & Interaction skills (diplomacy, intimidate, etc.))
Complete Feats
Rogues Gallery (loved that book! It would be great to see 1-20 progressions for each of the signature characters, Ember, Hennet, Kerwin, Naull, etc. and also sample characters from the various other books, like Morthos, the 'iconic' Warlock or Ialdabode, the 'iconic' telepath, as well as dozens of other sample characters, usable as BBEGs or other 'boss mobs')
A version of the Polymorph / Wild Shape mechanic that doesn't break the game and / or suck. Is it really that hard?