If I ran the zoo this would probably be the business model:
D&D Core: DMGs (maybe just one or two, as many as 3 ever), PHBs (the place to introduce new races and classes. They would need to be functional without anything other than PHB material for off-line players.), dungeon tiles, minis, occasional adventures, and Monster Vaults. These would have rare new releases but would pretty much stay in print, much as Essentials are meant to do. This would get people involved in the game and would supply accessories players want/need.
DM/Setting books: These would be evocative and well-written books targeted to DMs. Books like Demonomicon, Planar Handbook, and even setting books would fall into this category. They would occasionally go out of print as there would certainly be some point where noone else would really buy these books.
DDI: Primarily, this would offer lots of Dragon content (updated options for classes, new feats, new items, like what they currently do, but with even MORE content. This merges current Dragon magazine with all of the splat-books, and would get uploaded into the Character builder at the end of each month.) and Dungeon content (delves which incorporate dungeon tiles, full adventures, and new sets of monsters which would get uploaded into the Monster Builder at the end of the month, and House-ruling/DMing advice, subsystems, and other ideas concerning game design.) All Dungeon/Dragon material would go through the full R&D process, as they've recently shifted to. DDI would also include tools such as a Character Builder (on-line with easy off-line exportability) and a Monster Builder, minimum. PHB/Monster Vault info would make it into the builders a month or so after release.
Why: DnD maintains a presence in Brick and Mortar stores, though on a smaller scale than currently. There won't be constant new releases every year. They'd cycle through a new set of Dungeon Tiles every 4-6 months, and 3-4 new DM-focused books a year. Player materials could be released in DDI to coincide with DM-focused book releases, thematically. DDI would be the primary revenue stream using te subscription model. Most of the Game-designers would work on this project. The DM-focused books could likely each be done by one or two great writers with a minimal amount of design work involved. They could publish "Best of Dragon" books every year to get material into non-subscriber's hands. They could take Adventure Paths published in Dungeon and put out Deluxe Box Sets of the them complete with tokens for the monsters, pull-out maps, and the entire adventure.
This would piss off people who don't want to pay a subscription to play DnD. Most of the new content would be Subscriber-only. There would be enough book stuff to play, but not enough for those who like new crunchy bits to experiment with every month or so.
I'm no business guy, but I think DDI killed the Splat book sales. That doesn't mean DDI was a bad idea...it just means that focusing on producing books full of crunchy-bits entirely duplicated in the CB was a waste of time, and other books focusing on setting, adventure hooks, and such, could have been a much better venture. Not to say people don't WANT those feats, build options, and Paragon Paths...just that you should charge them for them via DDI.