Personally, I would love to see Dark Sun done well but that's not going to happen so soon after the Paizo botch-up of it in Dungeon and Dragon. However, I can hope, can't I?
Planescape would surely have to be a certainty for an update: there are so many fans and almost all of them seem to be vocal in their support!
Similarly, Birthright is simply screaming out for a huge update. Why not redo the whole 2E boxed set over, say, three issues and perhaps with some web content that is paid for separately?
More updates to Spelljammer would also be great and I would be happy to read anything on Al-Qadim, Mystara, Kara-Tur etc... even if I never used the stuff.
However, whatever is decided by Erik et al with respect to Campaign Classics, I really hope we see more and more stuff about the lower planes.