In 2007, Nintendo released "Super Mario Galaxy" for the Wii which received critical acclaim and has said to be one of the best Mario games ever created. The sequel, which was released in 2010, received even higher reviews and is possibly one of the greatest Wii games ever made. In January 2013, Nintendo announced that a completely new Mario game will be shown at the June E3 event.
Besides the vague announcement of a new Mario game, no specific details about the game were released. It's also important to know that this game won't be a "New Super Mario Bros." game which is Mario's sub-series. It's very possible that the new game could be a third installment in the 'Galaxy' series, but one sequel was enough. Nintendo normally doesn't do sequels to specific Mario games. "Super Mario 64" and "Super Mario Sunshine" never received sequels, but they changed things around with the 'Galaxy' series. Would a third installment in the series be too much? Not at all.
Nintendo could've actually planned the 'Galaxy' series to be a trilogy spanning 2 console generations, but this hasn't been confirmed. If an entirely new game was created, it would have to be revolutionary in the same way that 'Galaxy' was. Nintendo is completely capable of releasing a heavy-hitting Mario game for the Wii U, but we sadly won't know anything about the future title until June.
The next Mario game will obviously feature GamePad integration and will be extremely important in the game. Expect mind-blowing HD graphics in the next Mario game as well. The game will have Miiverse capabilities and could possibly integrate NFC. Wii remotes may be supported, but since the game will most likely be single-player, Wii remote play may not be available.
Gameplay may or may not be similar to the gameplay in both 'Galaxy' games, but Mario himself isn't expected to have any major changes besides the graphics. Will the next Mario game be set in the future? Will it be set underground? Are we looking at a 'Universe' game instead of a 'Galaxy' game? We'll have to wait until June to see.