interview with the Associated Press, Super Mario Bros. creator and leading Nintendo visionary Shigeru Miyamoto mentioned that the Japanese console giant is already working on a successor to the now two-year-old Wii U.
"For the time being, our focus is on the Wii U hardware, but Nintendo as a whole has groups working on ideas for new hardware systems," Miyamoto said. "While we're busy working on software for the Wii U, we have production lines that are working on ideas for what the next system might be."
Keep in mind, this sort of far-future planning is routine for console manufacturers. It would be surprising if the "production lines" Miyamoto mentioned have moved beyond the early planning stages. Don't worry, it's going to be at least a few more years before you have to work another Nintendo console into your budget.