One not too implausible scenario would be something a little like what just happened in Israel - a new Centrist party being formed. Let's say McCain and Lieberman decide to try and pull it off, and gather several Senators and Representatives - enough that neither party has a majority in either house. That will create some interesting situations... of course, there may well be funding issues for said Center party...
It's all long term stuff, and there a ton of obstructions that need to be removed to make it happen - including the fact that both the Democratic and Republican parties will fight against it until the bitter end (and their odds of coming out on top are very good). But you have the right idea for how it could start - a few noted figures breaking away from the big 2 and being willing to fight for their causes with less cash. In my dreams we have a few notable moderate Republicans move into the Libertarian camp giving the Libertarians some much needed credibility and a few noted Democrats move over to the Green party (or some other left leaning party, I'm not so sure of what is out there on that side of things). It would shake things up quite a bit and force the Dems and GOP to redefine (or simply define) themselves and give us something much closer to the 4 party system I noted above. The day something like that happens will be a great day for this nation - but I doubt it will make it much past my dreams in reality.