I like pj's four-party system, which would require a sort of schism on both the left and right. Only a short time ago that seemed impossible, but with the Republicans in such obvious disarray today (I'm starting to pity poor Romney trying to please everybody at once) and the Democrats facing internal pressure between pro-union forces and pro-business (or something -- see Chicago teachers' union vs Emanuel), we might be in that rare alignment when simultaneous schisms wouldn't be unthinkable. I've never seen a multi-party end result as impossible, I've just not been able to see the path to get there until now.
If multiple caucuses even formed within the two major parties, it could signal a big shift how they function. The moderate or business wing of the Republicans, for instance, really needs to organize to balance out the Tea Party elements so that in four years they don't again end up with bizarre documents like the party platform adopted at this year's convention and so that in the future budgets and debt ceilings don't bring the entire legislature to a halt.
(Although with the R's being their own worst enemy these days it's tempting to just stand quietly and watch them self-destruct.)
The third-party thing (most likely leaving the US with a left, right, and center party) is less satisfactory because with three elements in play, the smallest gets to be kingmaker over and over, something like the Supreme Court with Sandra Day O'Connor. However, in some countries (in Europe, and in Australia) the greens have emerged as a third party with considerable significance. It's hard to say the US is ready for such a development due to our sadly retrograde science literacy, but anything can happen over time.
As parties multiply, you get less of a simpleminded left-right array and more complexity and nuance, which better reflects the actual range of public opinion. Also, the impact of gigantic donors would be more spread out so that overturning Citizens United becomes less pressing.
What seemed impossible only a year ago is becoming more and more imaginable.