Bad guy main character? Definitely been done. Luke Skywalker is the ultimate bad guy. He's a member of a rebel faction and succeeds in overthrowing the government.

Certainly it's all about perspective, because the bad guy is definitely the valiant hero.

If you want to look for playing the "bad guy" you'll want to decide whether it's as a rebel (good guy at heart, like a Skywalker or a Robin Hood or a Batman) or as a truly evil entity. Playing as the actual bad guy, attempting to thwart the efforts of droves of hero-types, would certainly be different. Not to say that it's never been done either, but it is a bit more of a unique idea that isn't done nearly as frequently.