You know what could be fun? Running an evil campaign.
There are mechanical advantages. I've got the Book of Exalted Deeds, and I'd like a chance to try it out. There are a whole slew of monsters in the core manual that would be fun to use. Or, rather, use in a different way; I've brought, say, celestials into a game before, but always as allies or characters to be negotiated with. It'd be interesting to use them as antagonists, and, conversely, it'd be fun to use some of those evil monsters as allies.
And then, of course, there's always the Book of Vile Darkness. I have an unaccountable fondness for that book.
It'd also be a chance to run "the good guys." Not that I don't already do that--my campaigns tend to be fairly ally heavy, and I have yet to decide whether that's a bad thing--but in this case I'd be running the actual "heroes," rather than people who are intended to make the heroes look awesome-er.
On the other hand . . . I don't really mind playing bad guys on the DM side (hate it on the player side, but that's a story for another day) and the advantage to that is that, in the end, the bad guys lose. If I've done my job right, and the players have done theirs right. If the players are the bad guys, then either we're going to have to sit down for some serious discussion of assumptions, or the bad guys are going to win.