For myself, I'd like to see Redhurst redone. It got hosed by being released as a 3E book just as 3.5 hit, and for my money, it's one of the best settings to come out of D20, and is eminently portable. So long as their are still schools of magic (and maybe even if there aren't), it can be ported over.
I want a 4E version of A Magic Medieval Society. The book is an absolute must-buy for me.
Necromancer has already announced a 4E Tome of Horrors on their boards, but I also want to see their best 3E monsters repackaged, either as Tome of Horrors 2, or something else.
I would love to see the Book of Eldritch Might redone, but my guess is that so much is changing with 4E that a lot of it simply won't be applicable to the new game. Still, hope springs eternal. Chaositech and other Malhavoc books would be nice, too, although I don't hold out a lot of hope.
I'd like to see all four of Goodman Games' tournament modules converted, and maybe their top five selling DCCs before heading on to new stuff.
Either Necromancer or Goodman Games needs to give us Dark Tower 4E.
I'd like to see Clockwork Golem repackage all of their lost tomes PDFs into one volume and convert it to the new system.
I would likewise like to see a lot of Ronin Arts' and Patrick Younts' stuff converted.
I want to see Creatures of Freeport 4E, although hopefully a lot of that will show up in the 4E D20 Freeport Companion, whenever it hits stores.