GoNintendo.com has a few details of the game - it appears that massive boss battles and "gimmicky" stages are going to play a large part in the game's design. You can also expect a bit of violence, as items like machine guns will be useable by your character. The story is being touted as something for families, something both parents and kids can enjoy together - so perhaps its not too violent.
The best news of all is the game will feature four player co-op support. Details on it are still sketchy and we still aren't sure if both systems, the Wii and 3DS, will support it. We expect to see more from this game at E3 later this year and will keep you updated.
Source: Gonintendo.com