Anything you want. It doesn't matter if most people there don't know who you're dressed as, they're all about the costume. We wore obscure Suikoden IV costumes there two years ago and got fawned on because I made armor and swords and the others had all this detail on their costumes.
It is a much more mature crowd than any anime con you've ever been to, though, so be prepared. No squeeing fangirls running through the hallways trying to get boys to kiss so they can go "oo yaoi!" (and may I say, thank god for that)
Founder and President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com