Apparently other characters like Peach and Wario were considered as playable characters for New Super Mario Bros. U. During an interview with Takashi Tezuka, GameInformer asked about the possibility of new playable characters outside of the Miis, specifically about Peach and Wario. His answer was as follows:
“We did consider that when we looked at which characters we would use. We really came down on the side of using Mii characters rather than some of the characters you had mentioned. Through the technology of HD, you’re going to see more expression and emotion on the Mii character faces. Of course, we’re going to continue making Mario games, so you can maybe look forward to some of the things you were asking about in the future.”
It doesn’t seem like we got a direct answer, but we can rest assured that New Super Mario Bros. U will not feature anymore playable characters outside of Mario, Luigi, Blue Toad, Yellow Toad, and Miis. Unless, of course, Nintendo is keeping quiet on any secret unlockables.