There is not a LOT I need to by at GenCon. I hate having to haul stuff back home that I can buy back home (and at a discount at the FLGS). Therefore I typically use my GenCon budget for stuff I can't normally find elsewhere (or won't be out for awhile). This ends up being art, t-shirts, discounted stuff, etc.
Of course, that being said, I would love to get my hands on WitchHunter as well as any new Mutants & Masterminds products that come out, as well as anything they may have with the new Warhammer 40k game.