I’d like to see the Draconomicon series continue with a book for each of the following: Demons, Devils, Elementals (including Genies of all kinds, and Yugoloths (why are these so weak in 3.X?).
Of course, I’d like these books to be deep and detail much of the planes they exist on. Page count more in the Draconomicon range as opposed to the follow-ups. Oh, and with the glorious art as well.
Also, for those of you who have DMG II, I’d buy a nice hardbound book just full of the NPC writeups like they have there. It would be a huge timesaver (and I love timesavers).
Lastly, hardbounds on Arabian Adventures, Indian Adventures, and Mythical China.