Sigl: Yes without a question I definitely want a book on Sigil. Bring the cage back to life again, with a well illustrated, surreal and high-quirky concept books. Showcase the factions and guilds (I'm not advocating restoring Sigil to the way it was before Faction War, that would be uncreative). Make the Hive, Lower Ward, Lady's Ward, Clerk Ward, Market Ward, Guildhall Wards and Undersigil each distinct settings all in own unique ways. Offer plenty of hooks for more ideas... Even make some small tie-ins to Planescape: Torment to possibly draw in others. But emphasize the interesting ideas unique to planescape, and have plenty of stuff to use for all levels. Even ways to make low-level adventuring ideas such as gang-warfare, walking in the bad part of town or magic shop madness in Sigil interesting.
Campaign Setting (Limited Releases or One-shots): Of course you can always bring back Planescape, or Dark Sun, or any number of older settings. I'd certainly support them as a limited or oneshot release.
FCIII: Yeah I'm another voice that wants to see a book on the Yugoloths. They're certainly my favourite fiends, and the book should find some very good niches for these fiends to fill. Explain the lore about the ancient and mysterious Baernaloths, shine the spotlight on the General of Gehenna, the Oinoloth, Anthraxus, Charon, Bubonix, Apomps and more. Also throw in Gehreleths/Demodands, Hordelings and Night Hags since they're neutral evil fiends.
Celestial Codex: I want a book on the residents of the upper planes too. And I certainly want such a book to be more than just on the LG ones. I like the CG celestials a lot better, Paizo publishing seems to like them a lot better too, as you should notice how recently Queen Morwel and Gwynharwyf have factored a lot into the background of the Demon Lords. Such a book should be about all the celestial races (because I don't see there being enough support for this being a series) a celestial version of the Fiendish Codex, but parts of Book of Exalted Deeds II and Savage Species thrown in. It should cover how to use them in campaigns, how to use them without overshadowing the PCs, and even how to play them as an option.
Faeries & Folklore: Basically my idea of of the Fey book, cover not only the Fey, but also faerie-tales and folk-lore from all around the world in a D&D campaign. The book should cover faerie societies, the faerie world and what happens when they meet mortals, for good and ill.
Heroes of Intrigue: This needs to be a book, there needs to be a source for campaigns based more around roleplaying and character interaction. A look into playing with politics, intrigue, trade, romance and other things that don't involve swinging a sword around.
The Apocryphal Codex: This book should be about the LN, CN and N outsiders, the ones that don't quite fit in. It should be about the Modrons, Inevitables, Slaad, Rilmani and other LN, CN and TN outsiders. Even ones undiscovered before such as the mentioned in passing Aphanacts, for all those who for example want CN outsiders being something else other than giant frogs. This book should explain some of the ancient war of the Vaati, and the consequences it brought on all existence. It should detail Primus, Ssendam, Ygorl, Renbuu, Chourst, Centre-Of-All and more...
The Goblinoid Book: A book on playing goblinoids and orcs as player characters. I want this book to present many different goblinoid societies from the classic savage humanoids, to an enlightened parallel humanoid society, to social pariahs as goblinoids living in a ghetto in a human city. This book should for example provide you with compelling ideas for example if you wanted to play something like a LG Hobgoblin Paladin, and other different ideas.
Steamtech D&D: I want a book on WotC's take on steam-punk settings, specifically steam punk in D&D. It could be a D&D and D20 Modern bridge book, but I want there to be things for playing wizards and bards in a steam-punk world. Emphasize different aspects and parts for a setting that's more technologically advanced with examples of Victorian London, the Wild West, and even old worlds like other parts of Europe or Asia or Africa colliding with technological progress. Put tools in for a world with airships, strange wonders and inventions and new ideas and schools of thoughts coming out.