Many of these have been said before upthread and in numerous threads, but I'll lend my support.
Fiendish Codex III: FC1 is IMHO, the best book released in 3rd Edition. FCII isn't bad, either. I'd love to see a continuation of this line to give the yugoloths the full treatment, and include some of the other fiendish races. Basically, combine the best elements of FCI and Lords of Madness. Leave out the new spells and prestige classes this time (unless they are geared towards the fiends themselves, not fighting them), and devote more space to the monsters, their histories, and environments.
Planetscape/Manual of the Worlds/etc.: Plane-hopping has been given a very thorough treatment in various D&D resources, but world-hopping between different Material Plane planets has been given very little. Follow the toolkit approach of Manual of the Planes, and offer various ways for getting from world to world, including (but not limited to) spelljamming, travel via gates (Stargate anyone?), and spells. Differentiate between alternate Material Planes and different worlds on the same Material Plane. Describe some of the legendary worlds from D&D's rich history.
Campaign Classics: Mystara/Dark Sun/etc.: Give each of the old campaign settings a single hardcover treatment, being as exhaustive as possible on maps, prominent locales and NPCs, unique monsters, etc. Summarize which elements from different books are appropriate for the setting in question without using the valuable limited space with reprints or reinventing the wheel. DO NOT INCLUDE ADVENTURES!
Inner Planes Book: I can't think of a clever title for this book, but it would give the Fiendish Codex/Lords of Madness treatment to the elementals and the elemental planes. It would also bring back the paraelemental and quasielemental planes as options, along with some of the best of the paraelemental and quasielemental creatures from past editions. And of course, the archomentals would be included. (BOZ and I already did the work for you in Dragon Magazine

).
Order & Anarchy: Good vs. evil is well-developed, but how about giving Law vs. Chaos a go? Include information on the legendary battle between the armies of the vaati and the Queen of Chaos. Explain how to get away from the law=good and chaos=evil mindset. Include the slaad lords and the revised modrons. Describe the planes of law and chaos in greater detail. The book could be expanded to include neutrality as well, fleshing out the societies of the rilmani and other beings of neutrality.
Complete Epic: Overhaul or replace the epic spellcasting system to be easier to use and less easy to break, expand the advice on running epic-level campaigns, gather up all the updated rules from the various books and the excellent Epic Insights column on WotC's website, and don't mention Union. Include better synergy with Deities and Demigods and revise the rules for ascencion to better fit with the epic ruleset.
Forgotten Realms: Lands of Mystery: Delve into the other continents of Toril, such as Maztica and flesh out the rest of the world.
And for the eight, the demiplanes idea described upthread sounds great.