While it sounds like the BBEG has had time to remove most obvious obstacles from his path, you could always have a crown-prince or legendary war hero or something like that. This long-lost Good guy has been in exile for years or voluntarily went into hiding and has come back to the town in secret having heard about the BBEG. He sees what the BBEG is planning and starts working to subvert him, which is where the PCs come in. The PCs could either try to help the GG covertly, removing political obstacles to his return, or overtly, having the GG emerge as a political figure and acting as his bodyguards.

Just a thought, it sounds like a very interesting campaign. Political intrigue is very hard to do (I'm currently debating whether my campaign needs/wants it or can do without).

-JM