As toxic as Other M was, I wouldn't be surprised if Nintendo is just going to let the Metroid property simmer down a bit before figuring out what to do with it.
It's too soon for another Mario title, even if 3D World was the lowest-selling flagship Mario game ever. That Zelda button-masher (oops, I mean, "Dynasty Warrior") game is a thing too now, so I don't think Retro's working on Zelda either
It's not more DK either, I think that's obvious.
It's not F-Zero, because nobody will buy a Wii U, much less a game that will undoubtedly sell badly on it. Also because Iwata's well and truly given up on the hardcores.
Custom Robo? HAHAHAHAHAAHAAA.
Starfox? Hmm, it's the most likely out of the bunch, I think.
As far as it being some dumb-ass party game crap, I don't think Nintendo's willing to waste one of their biggest assets on another throwaway annual Mario Party installment. Well, certainly not stupid enough to, anyway.