Well, I've mentioned some of these before, but...
1) I'll second the Expanded Oriental Adventures. That said, I don't see any good reason that a third-party company couldn't do this just as well (couldn't include Kara-tur or Rokugun, but I don't really consider those huge losses.)
2) A Draconomicon-style book dealing with Humanoids. Preferably covering orcs, goblinoids, gnolls, kobolds and lizardfolk. This is distinct from a "Races of" book on the same races, since that would be unlikely to contain lots of pre-generated stats, warbands, and DM material, in favour of feats, prestige classes, and other player material.
3) Actually, more deluxe monster books in general would be good. These are some of my favourites from the past few years. I think fey and giants are probably the best remaining choices.
4) An environment-series book on planar hazards. The existing books are good, but they cover real-world environs, or fantasy versions of the same, but surely there must be more extreme conditions in the planes?
5) Likewise, more environment books in general would be good. I think forests and mountains would make good candidates, and there's probably even room for one on plains as well.
6) Adventure modules written by Bruce Cordell.
And my dream product, which is a massively powerful DM aide for the computer. But I don't think that's even possible in the current edition any longer - there's just too much accumulated lore.
I'm primarily a DM, and the single biggest problem I face with the game is that it requires too much preparation. The products I've mentioned are all DM aides of some sort. The simple fact is that I really don't need yet more feats, prestige classes, or other options for PC customisation (I don't object to them; I just don't need them). But anything that would make a measurable impact on the difficulty of preparing a game session (or generating new adventures) would be gratefully received.
