Surprise, surprise. According to the Paul Gale Network, a new 3D Mario game, separate from New Super Mario Bros. U, is in the works and is due for release sometime next year. The source goes on to compare the release of New Super Mario Bros. Wii and Super Mario Galaxy 2, suggesting that we will see something similar with the Wii U, only with a longer time span between releases.
You also might be wondering how much we can trust the Paul Gale Network. In late 2011, they were the ones to break news of the PlayStation 3 exclusive, Title Fight, now known as PlayStation All-Stars: Battle Royale. Whatever the case may be, we can certainly expect a 3D Mario game on the Wii U.