I see Thomas dismantling the White Court hierarchy—eliminating the Skavis and Malvoras (Malvori? Malvorae?) Pain and fear are pretty gross things to feed on, and through some kind of Machiavellian schemes, Thomas guts them.
Or maybe he pulls a Monsters, Inc. and gets all the White Court vamps to feed on joy, so they turn into the biggest philanthropic organization in the country! (Because that's going to happen).
I've heard the idea of him becoming a Knight of the Cross, and I'm iffy on it. It'd be cool enough if it happened, but I think that you need to be human, and Thomas isn't. Maybe Harry can figure out a way to exorcise his Demon or something first, but I don't think a Sword of the Cross could just purify him or something.