In a recent interview with French outlet Gamekult, Nintendo's spritely King of old man magic voiced plans to move away from headlining the Mario franchise -- "Creating a game of the caliber of Super Mario 3D World takes a lot of energy and there is little chance that I could take the time to work on the next Mario. However, I want to get involved again and more deeply in the development of smaller projects, but that doesn’t mean it will be a modest download-only title or something like that. It is not impossible for me to work on a new game as the main producer in the future, but even when I’m not directly in charge of a project, I always look at what the teams does in terms of gameplay, and I trust them. Don’t worry about Mario!"
There's quite a bit of potentially contradictory information there, and being translated from Japanese to French to English probably doesn't help either, but it stands to reason that Miyamoto may want to shift towards fewer marquee titles down the line. Nintendo publishes new I.P. all the time, but most people only focus on Mario, Zelda, and Pokemon. If Miyamoto were to jump on the next Xenoblade, Pushmo, or Pandora's Tower, it would likely get a lot more attention. I bet he's also getting tired of trying to solve the problem of making a game that appeals to multi-millions of people. The only way to go back to the tighter, simpler development style where he got his start is to go small.
Then again, Ozzy Osborne has been threatening to retire for over 20 years, and he's still touring. Miyamoto may talk about out on Mario, but time will tell if he can really pull himself away from the little Italian stereotype.