Historical Supplements. I would LOVE to see a product that was, say, 160 pages. And it had 20 pages or so for each of the folllowing historical "eras": Mesopotamia, Ancient Greece, Ancient Rome, Egypt, Ten Kingdoms era China, Persia, Celtic Europe, and the Byzantine Empire. Each would contain a short history, some detailed locations (no maps or anything), and a few ideas on how to implement into a fantasy scenario. Each section would also have a few pages of crunch (no more than 2 or 3 pages). Of course, all the locations are in the "ancient world", so it's possible to link them in some way... or even use the material to create a sort of "Alt earth" setting.
That would be really cool.
It's already been mentioned, but a 4e unearthed arcana/house rules book would be neat. GMs like to fiddle, and I think one of the reasons a lot of people are wary of 4e is that the game seems to strongly discourage house-ruling in a way that's never existed before. I don't think the designers intended that, but there's definitely a perception that is the case. So make a house rules book! It won't break the system.
Dungeon Delve #2 would be greatly appreciated, especially if it used templates and the like to create entirely new statblocks. Each should also include a short paragraph on expanding the adventure in a way that makes sense, similarly to how it was done last time, only with perhaps a few more concrete ideas.
And, since this is pretty much already a dream post already, may as well go full bore.... a book containing the SPELLTHIEF class. If wotc included a spellthief in 4e, I would be a complete, absolute convert to the cause.