Where will the game morph too?
I don't really know. I think there will be a lot of new classes (probably more then in 3E) over time, but a lot will be achieved also by expansion of existing classes (alternate class abilities, more powers, paragon pathes).
An interesting question is what kind of subsystems might still be coming. Many posters on these boards would like Craft/Profession subsystems, and I can see WotC eventually doing one themselves.
The current artificer preview shows there are still some mechanically option left unexplored, and there might be way more coming.
The Forgotten Realms book apparently introduce "backgrounds" as a new subsystem (it might be nothing more then a few special feats, but it could be more), and there might be more to come. Possibly even something complex as the conviction/willpower system in Exalted or other games.
There might also be new subsystems for non-combat stuff and especially skill challenges. I can see a lot of these things being introduced in the DMG 2.
Well, that's all overly optimistic, of course. But I prefer to be optimistic here...
Yep. This. It's only a matter of time. And let's not forget books containing new powers too since those seem to be the new "thing" in 4e.
From what I understand, powers, paragon pathes and possibly destinies will be the major parts of the "splats". And naturally, also be the things that can break the game...
Let's enjoy things as long as they are, and later cherry-pick the non-breaking parts!