duckroll was speculating that it might be Monolithsoft's secret RPG. I have to agree. The character designs are very Soma Bringer-ish. It may be by that team. Which sucks since Nintendo won't give us an English version of Soma Bringer.
They do this every year and people act surprised every year. Nintendo uses these conferences for a USA Today story not for NeoGaf. The fact that Metriod was even put in there was likely a last minute bone to people watching the stream.