Hrm. Eight products, huh? Well, let's say we define our focus first, shall we?
In any given year, we want to include both generic sourcebooks and those that are world-specific. The two current WotC worlds being Faerun and Eberron, let's give the same number to each - two. That leaves us with four (half) of our initial total for generic sourcebooks, while not neglecting either of the fanbases (despite my inclination to giggle and give Faerun a book ending the setting.

)
As for the generic supplements, I'm thinking we need at least one book on critters - the primary debate here is whether it's giants or fey. I prefer the latter, myself, but I suspect the former would be more useful to the majority of campaigns. So let's go with giants, and push fey back to next year.
For another sourcebook, fluff-centric (with a bit of tasty crunch,) how about we address the time-constrained DM with a book that provides interactable
things. I figure, we could go with places (towns or more specific locations, like taverns,) or groups. Because I think it's easier to pull things from the latter, I'll take a page from
Complete Champion's organizations, and put together a book of societies, guilds, and the like.
Moving on, let's put in one of the ever-popular "how to" type series, especially since they're among the most portable. Intrigue and mystery, as pointed out, are the two big ones currently open.
So, our fourth open product. Hrm. Let's do something big, shall we?
Generic Products
- Rock and Sinew - The Book of Giants: The fourth book in the Draconomicon/Libris Mortis/Lords of Madness series, this would cover giants, from the lowly ogres up to the storm giants and titans. Include cultural notes, notes on how the interact with the more humanocentric world around them, interrelations, and the like. Include options for giant fighters (though we all gnome dwarven rangers do it best
) as well as a small selection of feats for larger-than-normal characters, including some sort of "giantblood" template that provides for normal (human/demihuman) characters to grow so as to take advantage of the crunch bits included. Short (8 page each) appendices or web enhancements that detail differences between the presented material and how it fits into Eberron and the Forgotten Realms. 224 pages.
- Guilds and Fellowships - An Organizational Guidebook: Including a thorough overview on the affiliation rules, as well as discussion on how to create new organizations, this sourcebook presents detailed groups for the players to befriend or oppose. Divided by type, it covers thieves guilds, adventuring societies, mystical associations and military orders, at around 10 pages per. Include at least one map per organization, a half-page/column of plot hooks, notes on introducing it to the players, and prominent members (stats limited to name/race/class/level). Each organization should also include a tiny bit of crunch, such as a specific feat or two, substitution levels, or similar abilities. 224 pages.
- Heroes of Intrigue: Covering adventuring in political situations, this, the third of the Heroes of... line, addresses the fact that social situations can be just as conducive to adventures as dank dungeons or blood-soaked fields. Include discussions on the underlying themes of political campaigns, how to implement political overtures into an existing game, ideas for how the core classes (at least) tend to handle, and contribute, to political games. Make a point to cover uses, abuses, and intentions behind the social skills (Bluff/Diplomacy/Intimidate/Sense Motive,) as well as the ones that'll likely see more use in such a game, such as Forgery. Probably provide methods for either adjudicating the uses o such skills in extended opposed checks, as well as notes on experience provided for it. 160 pages.
- Greyhawk Campaign Setting - From the Ashes: Full-sized, self-contained Greyhawk sourcebook that pushes the timeline several years past the war, assuming that nations have begun rebuilding, thus providing a very open future for DMs looking to utilize it. Cover racial locations, nationalities, and the like, importing a lot of material from the Living Greyhawk Gazetteer. Include regional feats and a hefty section on how to adapt the generic, 'pseudo-Greyhawk' material presented in 3.5 books to the setting, when necessary. 320 pages.
Eberron Products
- Rising Nations: Sourcebook covering, similar to how Five Nations did, of the non-Galifarn nations; specifically, Darguun, Droaam, The Eldeen Reaches, Lhazaar Principalities, Mror Holds, Q'barra, Valenar, and Zilargo. Almost no crunch, not even the bit located at the end of the nations in Five Nations - we've got a lot of material to cover, and very little space to do it in. 160 pages.
- Planes of Eberron: Planar sourcebook written by Keith Baker that discusses the specific role of the planes in Eberron, including touching on their relationship with the moons, manifest zones, coterminous/remote information, and planar descriptions using the format presented in the Manual of the Planes. Specific focus on extraplanar adventures in Eberron - why do the locals choose to go somewhere else? Do extraplanar beings come to them? Who can do it? 224 pages.
Forgotten Realms Products
- Beyond Faeurn: Stealing an entry from Razz, this would cover the three primary non-Faerun locations on Toril: Kara-Tur, Al-Qadim, Maztica. Location information, advice on running campaigns either set in those locations or short journeys there from a more Faerun-focused campaign, as well as adaptation notes to adapt mechanics to reflect the feel of the lands (eg, "arcane casters in x are more likely to be wu-jen, CA, then sorcerers or wizards). 224 pages.
- ? I'm honestly not sure what to place here. I'm reasonably familiar with the FR, but not with their recent product line, so I'm really a bit unsure of what areas haven't been covered. Regional sourcebook of some kind, though, located much closer to home than the above entry to appeal more to the fans who like their Faerun right where it is, thank you very much.