Okay I made a joke about 4th edition. In all seriousness what I would really like to see would be round rooms for dungeon tiles. Also narrow (5 foot) corridors for dungeon tiles.
Aside from Fiendish Codex III, the forest/jungle environment book, and Complete Butt-Kicker... the two part series Lexicon of Law and Lexicon of Chaos, the Books of Axiomatic/Anarchic Outsiders.
Add me to the count for a FC III. I'd also like to see an axiomatic creatures book, a giants book, a fey book, another planar book (detailing specific places such as Sigil, City of Brass, Dis, etc.- I know they have been discussed a little but I'd like a whole book devoted to planar cities ), and I wish they would revive the DM's kit (I wanted a TPB DM's Guide ).