My personal guess:
Wii Music 2 will show up, albeit possibly under another name, but there'll be enough good stuff so that we can laugh at it comfortably in retrospect.
Say what you want about Wii Music, it sold over 3 million copies. If "Wii Music 2" built on the original formula, maybe giving more control over the orchestra and whatnot, it could become this gen's version of Mario Paint Composer. I'd hardly mind that.
Also, my fingers are crossed for an asymetric Mario Party. If this game I've theorized about turns out to be real, it will be the first time since roughly 2002 I've cared about Mario Party.