What do you think the next UNANNOUNCED Mario game will be...?

[Aielyn]

Next Mario game in general?

Mario x Sonic, the third "Sonic" game from the Nintendo/Sega deal. Probably for release in mid-to-late 2014 (say, September).

Next game entirely in the Mario universe?

Mario Party 10, probably for release late 2014 or early 2015.