Manual of the Planes, of course, with a strong emphasis on Sigil and the Outlands, and more depth of the gith races.
Forgotten Realms Campaign Setting has been my favorite of the main setting books. If they can find some way to combine the flavor of the grey box and the production of the 3E book, it could be something special.
Deities & Demigods got skipped with 4E. I'd like to see it combine the traditional approach, with the various mythos, but have a large section for "epic" monsters.
As an aside, I like Weapons of Legacy but I don't see why it needs an entire book. It seems like it would do well tucked into the DMG or an Unearthed Arcana.
Speaking of which, Unearthed Arcana, but I'd like to see it as a kind of "options gone wild" book - a kind of combination of the PHB and DMG, but an emphasis on "fringe" options.