Well, we already saw Petey in one of the previews. He looked like some kinda...space...dinosaur....flower...thing. But it looked like he played a minor role.
I'm going to go out on a limb, here, and say that Bowser is definitely going to be the final boss. Only, he'll wear some metallic space armor and have glowing eyes...with a jet pack.
I'd be pleasantly surprised if it was someone else, though.