Well, as Nintendo currently does not have any Western RPGs, a revival of Raven Blade might be nice. Game looked pretty good (graphically at least) on the Gamecube before Nintendo pulled the plug.
I would also love Retro to make a game set in the Metroid Universe that has NOTHING to do with Samus. They introduced a lot of characters in Hunters, and as badly panned as Other M was Anthony Higgs has a surprising fan base. A Sci Fi shooter using Galactic Federation soldiers might be fun. Make their commanding officer a woman to poke fun at Other M while you're at it.