Here's my idea for a Mewtwo trailer, if Mewtwo does indeed get one to himself:
Basically, it starts out a dark cave, very small room, the outline of a creature standing in the middle of the room. This creature is of course Mewtwo, recollecting his memories of battling in Melee, which will show some black & white flashback gameplay of Mewtwo in Melee. Mewtwo mutters to itself that it was weak, that it went too easy on its opponents back during said time, which will lead to footage of Smash Bros 4. As this footage plays in and out, Mewtwo will say "This time...I will show no mercy!" as its eyes open, glowing blue in the darkness as the cave starts to shake violently. The camera will zoom out to the outside of the cave, which will explode in a powerful psychic-energy explosion as Mewtwo soars off into the sky, AKA a scene designed like the explosion of Giovanni's Gym in the First Pokemon Movie.
The screen then jumps to Mario, Link, Pikachu and Kirby, the main four fighters from Melee's intro and Brawl's reveal, battling on the Battlefield, this is when a giant black ball of energy will fall from the sky, crashing into the stage and blasting all four fighters off the screen. Mewtwo will then land on the battlefield, the camera zoomed up close so that it could only see its feet. The camera then pans up to Mewtwo's face, its eyes flashing as the title "Mewtwo Strikes Back!" flashes on-screen.
Mewtwo shows off its powerful new fighting style, displaying how much its attacks have been upped in knockback, how much heavier it is now, and more. The trailer will end of course with Mewtwo crushing three different opponents in a powerful Psystrike Final Smash, no Mega Evolution needed. The last scene will be a small CGI clip featuring said four main characters, piled on top of each other as they were knocked out, the camera panning around to Mewtwo as it held a Smash Ball in its hand, saying "I...have returned" crushing the smash ball and ending the trailer.