yeah, Fire Emblem and even Metroid are "niche" titles. Mario, Mario Kart, Pokemon, Smash, Zelda are still the bread winners for Nintendo. Everything else is minor league.
I wouldn't be so sure of that. Fire Emblem Awakening/Fates did a lot to change the mindset, and outside of Shadows of Valentia (which was more of a remake for the fans); it has done well over here.
Not to mention, as Quinn said..it has made a splash quite well overseas.
That said, I do think Metroid Prime 4 is the one that'd make a lot of sense to be revealed. It'd be very much appropriate for the audience that TGA gets; and has been one of the games that has been quietly in the shadows.