Super Mario Galaxy 3U is a game for the Wii U. It is being developed by Nintendo EAD and is set to be released some time in 2013. Some features have been revealed by Nintendo. The game will have a two screen mode and a controller mode. In the controller mode, the player can pull mario to a pull star by tapping and holding the star on the controller screen, in two screen mode, you can do this, and use the original method (using wiiumote sensor). The game will be the first 3D mario gane to support true multiplayer cooperative play. Player 1 (Mario) and Player 2 (Luigi) can compete in special arenas designed for multiplayer or do the story mode together. the tv shows a split screen, but the controller only shows your character (you must have two Wii U controllers). Of course, if you want to play the original form of multiplayer (you're a star that collects stuff), you can, even 4-player games are possible if three people become stars alongside Mario (you need three wiimotes to do the original form of multiplayer). The game will support online play using the Nintendo Network service. Players can compete against other players from around the world.
Here are some of the online modes: Shard Collector: Try to collect the most Star Bits before the time runs out. Cosmic Battlezone: Defeat your opponents by using power-ups on them (The Games Power-Ups are still a mystery). Star Hunter: Player's vote on a map, then it's a race to see who can find a Power Star that has been hidden on that map, first person to collect it win's the round.
Those are all the confirmed Features
The plot is still unconfirmed.