I would love to see some of the following:
Old non-D&D modules/sourcebooks updated to d20 status. This would include the old Fantasy Master series of books like Aesheba: Greek Africa, Town of Baldemar and their line of modules. Central Casting updated to d20 including Dungeons and Heroes of Legend. Old Runequest/Midkemia Press bookls like RUnequest Cities, Carse and Tulan. Of course, what talk of old products would be complete without a mention of Bard Games and their ilk such as Atlantis triology and the Atlantis Lost World series it latter became? Of course, it seems too late for Talislanta and Arduin, both of which would've made perfect d20 settings with their unique take on things.
I'd love to see WoTC resurrect Role Aids as a seperate official line that handled all the optional material and was OGL. Their sourcebooks, like Dragons, Giants, and Monsters of Myth & Legend (I,II,III) were setting standards that TSR sometimes failed to equal.
The old Primal Order capsystem series needs to seriously make a comeback. With the OGL and d20 systems being out there, and numerous new game systems that Primal Order never covered, this could be a really great seller. Their line up of potential books still haven't been covered, The Military Order and the Economic Order, would make fantastic sellers.
No mention of a generic sourcebook could be complete without Citybook and their printers, Flying Buffalo. I'm very pleased to hear about Grimtooth's Traps for d20 but hope that other material makes it as well. The Hole Delver's Guide was humours, but there were some serious entries in the stable as well. Of course, Lejentia would make such a serious d20 sourcebook, its not even funny.
There are some other products I'd love to see updated. The Dungeon Masters Design Kit. I lent mine out to a friend and it was destroyed, lost like Atlantis under a sea of burning cola and hi-c. The various DMG books in Blue from 2nd edition. The first and fifth ones, Campaign Sourcebook and Catacomb Guide did a great job on starting GMS, while Creative Campaigning, did a good job on more senior material.
Of course, the 1st ed DMG still gets some use. Tables for everything. Information on translatting characters from one game to another. (Hell, I was just complianing that d20 official games need something like this), and lets not forget the simple section on traps, gems, jewelry, and gambling.
Looking over this list, it would seem that I'm looking for more books for GMs. Not more monster books, not more magic items, but more how to advice and metathinking on why and how things should be done in a certain order.
Tables And Tools... yeah, that's the ticket. A new sourcebook for GMs of all levels...