Honestly, I don't think there's a scenario where he can become Prime Minister after 2008. He's really burned his bridges with the federal Conservatives by this point, to the extent where they would never elect him as their leader, and it's not like he's exactly moderate enough to switch parties and try to work his way up as a Liberal or a New Democrat. And even if he decided to try and go the extremely difficulty route of starting his own party and trying to make a breakthrough, at this point he would really struggle nationally. Most right-wingers are unlikely to vote for him after his anti-Harper campaign and he's never going to do well in Quebec. Theoretically he could sweep the Atlantic and maybe force a merger with the Conservatives, but that's unlikely. I just don't think this is particularly realistic after 2008.
On the other hand, If Harper loses the 2006 election and never becomes Prime Minister (which actually seemed likely when the election began and, really, up until the end of December), I think Williams could be in a good position, particularly if he remains a thorn in Paul Martin's side (which I think is a reasonable assumption). Obviously this would endear him to conservatives and increase his prominence within the party, particularly since he's the type who really tries to make his voice loud and heard. If Harper resigns as leader and the party waits until, say, sometime in 2008 to choose his successor, I could see Williams having a good shot at succeeding him.