Hmm...
For MP9, a bunch of Retro minigames should appear, as well as some more Wii-mote interactive minigames. The Puzzle minigames could also make a return, and they should have 4P versions as well (for Mario's Puzzle Party, that's easy; for the others, I don't know how they would work).
They could also re-introduce Item Minigames (MP2 and 3), and maybe a Boss Battle compilation could work as well.