A Forgotten Realms Manual of the Planes - detailing all the planes and encounters of the Great Tree Cosmology.
Swampy Kingdoms - detailing the Batrachi creator race, their descendents, ruins, magic, history and other lore. What Serpent Kingdoms did for the Scaly folk.
Sylvan Kingdoms - detailing the Sylvan creator race, the Plane of Faerie, fey magic, encounters, fey crossroads, relics and ruins, and the Fey pantheon.
Soaring Kingdoms - detailing the Aearee creator race, and their descendants: the aarakocra, the tengu, kenku, and dire corbies of the underdark. Ruins, magic, flying cities, the Aearee pantheon, tree fortresses, etc.
A Kara-Tur sourcebook, revised heavily with new lore integrating more into the history and feel of the Realms. Asian themed, but not quite so much a rip-off of Earth's Asia. Mythological asia, with powerful dragon kingdoms, oni, ogre magi, giant monsters roaming the countryside. Krakentua. An order of monks that wear giant animated battle armor (or use spells to grow to colossal size) in order to combat the giant monsters and safeguard the villages of Wa, Kozakura and Kara-Tur. That sort of thing.
An Al-Qadim sourcebook: Same as above but for Zakhara. With lots of genies.
Fiendish Codex 3: Yugoloths, Hags, Demodands and the planes of Gehenna, Hades & Carceri
Celestial Codex 1: Scions of Celestia
Celestial Codex 2: Armies of Arborea
Celestial Codex 3: Champions of Right (Elysia, Bytopia & Beastlands)
Codex of Law: Mechanus, Acheron & Arcadia
Codex of Chaos: Limbo, Ysgard, Pandemonium
Codex of Concord: the Outlands
Inner Planes sourcebook (more genies & elementals)
Transitive Planes sourcebook(s)
Sigil Sourcebook
Western Heartlands Sourcebook
Cold Lands, Vasa & Impiltur sourcebook
Old Empires Sourcebook
Sword Coast & North Sourcebook
Adventures galore set in the Realms
NPC book, chock full of NPC's of every level for easy dropping in to any campaign.