Hmm - well one thing that I'd consider doing is a complete revamp and repub (not 4.0) of the existing 3.0 and 3.5 information - cleaning up all of the errors and packaging everything differently. Instead of packaging new feats, spells, PrCs, etc. in each book, consolidate like information (i.e. a "Feats of the Realms Compendium", "Faerunian Professions" - all the new PrCs, etc.) The empty space in each of the regional books could then be used to add more background information - rumours, plot hooks, more details on locales, ruins, etc. I think that maybe Mysteries of the Moonsea will turn out to follow this type of idea - we'll see.
The above pipe dream aside, I'd consider publishing a book on caravans, trading, and commerce; a new FR book on religions that would include things like the 2e trinity of books, updated and further detailed - priestly ranks and titles, typical cermonial garb, affiliated priest, nun and monastic (as per medieval monk and not D&D Shaolin Monk style monks) orders, details of the "feel" of a particular religion, etc.; a new FR book on regional militaries and mercenary orders with full ORBATS and TOEs;
Actually, regardless of campaign setting, I (as a time-limited DM) really want the little details. While generic information is good, I use FR and would publish for that - trade routes and trade goods - which ones are commodities.
Other than providing updated information on Maztica, Zakhara and Kara-Tur, If I could only release one product for the entire year, it would be an updated Forgotten Realms Interactive Atlas - the full world, retaining the parts that were removed for the 3e map, but using the 3e map style for political maps AND also including physical, climatological, and topographical map overlays. I (as a DM with little time on my hands to come up with the information on my own) would really really like to know the average temp, rainfall, etc. for a given location. Heck, all I really need to know is (since I use Weathermaster), the generic climate classification - is it sub-artic, temperate, etc. I'd love to know how tall a given mountain range is, how deep a given lake is. What this does is it *allows* me the ability to easily determine these things, and, more importantly, it allows me the ability to actually use information in the environment books - if I *know* how tall a given mountain range is, I can then apply information from Frostburn...
