The Official Rumble U site has updated with a number of new screenshots from the game. The screens show a number of different things from co-operating with other players to fight Groudon, to receiving a new toy as a reward. One screenshot also shows the GamePad as the players icon (the others are all CPU), a possibility of being able to play off TV during 4-player co-op?
The game is set to release for the Wii U in Japan this Spring. We will continue to update!
(Apologies for the weird formatting on this post, having so many images in one gallery seems to mess it up)