Gotta echo some of the above ideas. My list:
Heroes of Steampunk
Book of Gear - book full of useful items, kits, and alchemy. Great for low-level or low-magic games. I'd like to see a simplified encumbrance system too.
Feat Compendium
Heroes of Intrigue - this needs some politics too
Races of the Green - book of goblinoids
Monster book on constructs and oozes
Unearthed Arcana 2
Planescape and Spelljammer campaign settings
I'd also like to see WotC release more open game content. Some of their ideas could be so easily expanded on by third-party publishers, like relics, touchstones, and some of the feat categories.
To deal with the problem of needing so many books, I'd like to see WotC products come with a pdf cd. The pdf would be formatted for printing out individual prestige classes or whatnot. Feats, spells, and magic items could even be formatted to print as cards, like EN Publishing's Fantasy Money. This would also be great for making compiling all of material for a DM's campagin.
Oh, and I'd love to see some kind of experimental 4e-ish product where the game would be redesigned with no regard to the sacred cows of D&D. Like on his blog, Sean K Reynolds was throwing around ideas for reworking the item creation system. Maybe borrow some of the best ideas from MMOs and other RPGs. And it'd need a all-star cast of designers working on it too.