I think we can divide wotc's publications into 3 groups.
Core and core-like
Thematic
Speculative
There's only so much "core" and, lets coin a new term, "core-like" material to be written.
I consider the PHBII, for example, to be "core-like." It mimics the feeling of a core book and advances the same character themes as the player's handbook. Also in this category would be the Complete series. These are books that, in a way, there's no particular reason not to allow. Sure, they're not perfect, but they're about as well written as the core rules themselves. They don't really change the flavor of the campaign world any through their addition, and they provide a solid amount of crunch for players who want more options for their core class characters.
Then you've got your thematic books. These are books like Lords of Madness, that pick a theme and build material onto that theme. There isn't much reason to buy one of these books unless you intend to focus a campaign around them. Stormwrack, Heroes of Battle, Races of Stone, these are all books organized around a central theme and which, by their addition, tend to move your game in a particular direction.
Finally you've got your speculative books. These are books like Tome of Magic, the Incarnum stuff, anything organized around a new mechanic. Tome of Battle looks likely to be another one of these. They are only slightly unified in terms of theme, only so much as the new mechanic is, and they're organized not on grounds of flavor like Races of Stone, but on a mechanical principle like Magic of Incarnum. These tend to be a little erratic: because they're covering new ground mechanically, balance often isn't guaranteed right out of the box. Further, many players do not like these books because they fear new mechanics. Look at the furor over whether the Warlock is balanced, as an example (a fun example of a speculative piece of work in an otherwise core-like book). Still, amongst those who adopt these books, they tend to be tremendously popular.
My predictions are thus: there's only so much design space left in the core-like category. The same is true for the thematic category. These aren't exhausted by any means: people are still clamoring for their Heroes of Intrigue book, or a good "Heroes of" book related to thievery. We've covered most of the geographic locations, like the frozen north, the ocean, the desert, etc, but the genres aren't completed yet. And WOTC seems to be getting better at making up their own genres rather than relying on other people's: the book coming out soon on Dragon magic is a good example of home made genre creation. So there may be some new material to be had there that I can't predict.
The speculative category, however, is still wide open by definition. I'm happy, because these tend to be the books I enjoy most, because I like playing with new ideas. They're like new, intricate little toys.
So, I predict a slowdown in Core-like publications, but a steady stream of Thematic publications, with a side of Speculative books for nuts like me.
And eventually everything will reset with a new edition, but I think we've a ways to go with this one first.
