1. New Star Wars RPG books, any of them (preferably Ultimate Battlestations,, a Rise of the Empire/Clone Wars sourcebook and, a New Republic Era Sourcebook, a revised/updated New Jedi Order sourcebook, a Tales of the Jedi/KotoR era soucebook).
2. Oriental Adventures 3.5, going back to Kara Tur as a default. NOT using the Complete Warrior Samurai also.
3. A low-magic fantasy setting for D&D, instead of going over the top like Eberron, try scaling back to lower fantasy, with PC's not having fortunes of magic items and not having Wizards on every streetcorner. A setting where a PC is more than the sum of their stuff, Clerics perform miracles instead of healing being "resource management" after a fight and Wizards are something to be feared.
4. Pseudo-Historic D&D. Similar to the old "Green Book" suppliments, a book of historic eras and how to play them as D&D games, including setting-appropriate magic rules for them, and not assuming that it's as high-magic as typical D&D.
5. Dark*Matter d20 for d20 Modern.
6. Planescape Campaign Setting. We've had bits and dribbles doled out over the years with MotP, Planar Handbook, some Dragon Articles, this and that. If they can make a niche subsetting like Ghostwalk, they should be able to make Planescape.
7. Dark Sun Campaign Setting. Bring back a setting where psionics are integral instead of bolted on or designed to be ignorable (among other merits).