If Mewtwo gets a trailer, I have a couple of ideas for one.
If Ridley's included
-Samus in powersuit battling Ridley. (Ridley isn't confirmed at this point; I could see that happening JUST to mess with fans)
-Ridley gets Samus into a pinch.
-Shadow Ball flies outta nowhere and knocks back Ridley.
-Pan over to Mewtwo (CONFIRMED!)
-Ridley gets back up, roars/taunts and is confirmed at this point.
-Engage moveset clips!
If revealed alongside every unshown newcomer/vet at once.
-All newcomers revealed alongside returning vets.
-All out riot!
-Battle interrupted by dark swirling thundercloud formation.
-What appears to be a glowing blue sphere descends from the sky.
-Blue barrier breaks and Mewtwo surges forth like a badass.
-NOW YOU FACE MEWTWO! (That wouldn't actually be said, but that would be the feel at the time, essentially)
-Battle suddenly becomes Mewtwo Pilgrim vs The World
-Moveset clips start up, showcasing Mewtwo fighting everyone in a series of different clips.