We're barely past the 2006 mid-terms, but it's never too early to engage in pointless electoral speculation.
My prediction: there will be a surprisingly successful insurgent candidate on both sides... they might not win, but they will give the front-runners a bad scare.
Why would there be an insurgent on the Republican side? There are three major candidates today: Giuliani is a moderate, although he conducts extramarital affairs with a certain liberal flair. McCain isn't a moderate, but he's perceived to be one because of his maverick streak. Romney is trying to sell himself as a religious right guy, but the real religious right is not going to buy him. Even if they get over his Mormon faith, they won't get over his abortion flip-flop or his gay rights record. So we have three major candidates that are hogging up the center when the real energy in the Republican party usually comes from the right. That's a big space for a conservative candidate to run in: Brownback might get the conservative mantle, or it could well be Arkansas governor Mike Huckabee.
Why would there be an insurgent in the Democratic race? Well, insurgents are a little more common on that side. Furthermore, the left blogosphere is more organized than the right, and they are able to make insurgent candidates plausible. In 2004 it was Dean -- the internet raised around $50 million dollars for him. Who will it be in 2008? Not Hillary -- she voted for the war. Not Obama -- he is not strident enough for the netroots. If Gore runs, he will be the overwhelming favorite of internet left -- he would easily raise $75-100 million online, enough to let him to go toe-to-toe with Hillary or anybody else. However, Gore probably won't run, and Feingold has said he's not running. Wesley Clark could well wind up being the progressive favorite.