I bet it's just squid jump
I could totally see it being one of the Splatoon games.
But my money overall is that Nintendo's just going to do something easy, like a puzzle game (I mean, there's technically Pokemon Shuffle, but I think that's separate from this DeNA deal). My money's on a Candy Crush-esque Dr. Mario, or on a brand new puzzle game based on Animal Crossing.
Mario's a good way to make a big splash quick, but Animal Crossing seems to be Nintendo's new "Experimental Series." Plus, it's one of the Top 5 franchises for them right now, the brand fits mobile gaming, and unlike Mario, they're less afraid of tarnishing that brand.