Like many people before have stated, I would love to see some unique type combinations, instead of another bunch of Bug/Flyings and Rock/Grounds. Gen 5 introduced some cool types, including Ground/Ghost and Fire/Bug, but I would still like to see:
Ice/Fire. (SR, I know, I know, but in-game, the stab Ice Beam and Flamethrower could be deadly)
Water/Fire. Not sure how they'd pull that off, but an interesting combo anyway. Maybe a...Flaming Piranha, perhaps? :D
Electric/Fighting. I have an image in my head of a little demon man sitting on a big thundercloud, similar to Lakitu's from Mario games, haha
Grass/Dragon. This could be an interesting type, and I was hoping it was going to be Serperior's type before Black and White came out, but GameFreak let me down. This could possibly be Xerneas' type also, although it does look like it's part steel type.