So many options...
Fiendish Codex III: Mercenaries of the Waste - A detailed guide to the yugoloths, gehreleths, hordelings, night hags and other miscellaneous NE evil outsiders that have been left by the wayside in 3e, giving their histories, physiologies, leaders and stats for them that don't suck (finally!)
Heroes of Crime - Covering how to run a criminal-themed campaign, with ideas for running heists, cons, organized crime, vigilanteism, and even detective games for a taste of how the other side works. In the style of Heroes of Battle and Heroes of Horror, with a few PrCs, organizations, feats, spells and monsters being thrown in, but not the primary focus of the book.
The Age of Worms Adventure Path Hardcover - Or rather, I'd give Paizo Publishing the license to do it.
Complete Psionic 2: The Psionicing - Psionics desperately need more support, and Complete Psionic was not really what we needed.
Complete Stand-Alone Systems - Similar in theme to Complete Psionic, this book would throw a bone to everyone who loved incarnum (I know you're out there somewhere), binding, shadowcasting and truenames. Features a truename DC system that doesn't suck!
Also, in terms of miscellaneous decisions, I'd cull the needless fluff from PrC writeups and use the correct (read - 3.5 MM) statblock for monster entries (the DMG 2 block can remain in use for NPCs and templated examples).
Demiurge out.