Hmm...
1. Adventures - stand-alone or linked. Generic with adaptation data for Greyhawk, Forgotten Realms and Eberron would be ok, but I'd like to see world-specific modules too.
2. A "niche" product describing how to set up a working economy (I'd like it for Forgotten Realms but generic would be just dandy as well) - trade routes, how to set up caravans, cost per mile for land and water travel, expansion of the information in FRCS regarding imports/exports.
3. A "niche" product describing in full detail the physical geography of Forgotten Realms - size of the planet, axial tilt, etc. It would have a full-color climate map with lat/long lines and time zones, and a smaller series of regional topography maps for every region of Toril - including Maztica, Zakhara and Kara-Tur.
4. A working mass combat system that allows for rapid resolution of battles while also allowing for PC/NPC interaction with the masses of soldiers.
5. A new City of Greyhawk set, or decent Greyhawk support in general.
6. 3.5-ized re-issues of 1st/2nd edition adventure modules.