Hmmm, I'd imagine it would be a game where he fights and destroys all the villains he's been forced to play second-stringer to throughout the entire series: Metroid Prime, Dark Samus, SA-X, and an extra dose of punishment to Mother Brain.
There'd be flying segments, RTS battles where you control armies of Space Pirates, and Godzilla-style deathmatches against the main villains. Oh, yeah, and co-op and competitive multiplayer featuring Kraid, Phantoon, Draygon, Mecha-Ridley from MZM, and others that I can't think of right now as playable characters.