More planar stuff would be great. Not another Planar Handbook, but more detailed write ups on the Planes like the FR region books, complete with locations, monsters, and stats for the important players in the Planes. Maybe like Manual of the Planes II, or the Codex of Infinite Planes. And a Far Realm sourcebook.
Epic level anything. Stats for epic level versions of the demon princes, for instance, more along the lines of Dragon than Fiendish Codex. Expanded Epic Level Handbook?
A revision of the Deities and Demigods, like the XPH - maybe Expanded Deities and Demigods? Rules that take into account epic levels, and more "balanced" and useful salient divine abilities.
Powers Compendium for psionics. With more powers.
More Fiendish Codex books and similar themed ones, like... Celestial Codex, Axiomatic Codex, Anarchic Codex, and... Neutral Codex. Basically books on the alignment-theme outsiders, like the modrons.
Expanded Oriental Adventures with a more generic setting that encompasses all the material (or most) inside, instead of a Rokugan one. Not that I dislike Rokugan, but alot of the material is not usable in Rokugan by default.
2E setting books updated and revised for 3E, in the vein of the FRCS.
More creature books like the Draconomicon. Maybe the Book of Magical Beasts, the Book of Constructs, the Book of Fey, the Book of Monstrous Humanoids, the Book of Oozes (if all else has been done), and the Book of Animals, Plants, & Vermin (if there isn't much left to do)
An environment book on... outer space. Black Hole: Mastering the Perils of Outer Space & the Phlogiston.
Another variant rulebook like Unearthed Arcana.
A rulebook that deals with the gestalt rules in UA, and that expands on it to create prestige classes for gestalt characters, and how to run a gestalt, high powered campaign.
So to sum up it would be:
- Manual of the Planes II/Codex of Infinite Planes, and hopefully
-- Inner Planes Sourcebook (or one for each plane!)
-- Outer Planes Sourcebook (or one for each plane!)
-- Transitive Planes Sourcebook (or one for each plane!)
- The Far Realm Sourcebook
- Expanded Epic Level Handbook
- Expanded Deities and Demigods
- Powers Compendium
- Fiendish Codex II (which is coming right?)
- Fiendish Codex III (yugoloths)
- Celestial Codex I (eladrins)
- Celestial Codex II (archons)
- Celestial Codex III (guardinals)
- Anarchic Codex (slaad)
- Axiomatic Codex (modrons)
- Planescape Campaign Setting
- Spelljammer Campaign Setting
- Dark Sun Campaign Setting
- Birthright Campaign Setting
- Al Qadim Campaign Setting
- Expanded Oriental Adventures
- My Little Pony: the Book of Magical Beasts
- Robots in Disguise: the Book of Constructs
- Airy Fairies: the Book of Fey
- Animal Heads on Humanoid Bodies: the Book of Humanoids & Monstrous Humanoids
- Slime: the Book of Oozes
- National Geographic: Book of Animals, Plants, & Vermin (if there isn't much left to do)
- Black Hole: Mastering the Perils of Outer Space & the Phlogiston
- Gestalts & other High Powered Games