I was just wondering if you could do this. I want to join a game (preferably online) but don't know where to start, so it would be nice to be able to find games with given themes, for example, X-Men, Avengers, Teen Titans, etc. I also don't mind creating my own character--so long as it isn't in a randomized, unequal way--but the hosting universe should be at least somewhat familiar.

I don't think anyone has catalogued existing online games let along organizing them by categories. Basically, the way I have found games since I got back into this game is to watch game announcements in this forum. There are often people looking for players, or sites with ongoing campaigns and open door policies. It's just a matter of waiting for one that fits something you'd like.