I think the third Pokémon in the Reshiram-Zekrom-? trio will either be Water-, Rock-, or Ground-type. This is because Tsutarja is weak to Reshiram and Mijumaru is weak to Zekrom, so Pokabu should be weak to the third one. I know this is like a completely invalid argument, but I'm just putting this out there so when Black and White comes out, I can see if I'm right or not.