What about a trailer in which Mew2 is presented as "mew2s return", then he is attacked out of nowhere by Ripley, presnting both characters foghting each others
if you do it like that, then i think it should be where generic fighters are on pyrosphere, then Ridley pops up and the players start running away or attacking him, but then Ridley actually flinches, before getting angry and "Ridley enters the fray" should appear.
then it shows gameplay.
then at the end.. a silhouette of Mewtwo appears... on a notebook... Greninja's notebook, (i dunno) he then crosses it off his "list"
Lucario and Charizard are revealed to be next to him, and they all start skipping and dancing in glee, celebrating the second loss of Mewtwo, however Pikachu stays behind, and having been there and helped him in Melee, (flashback) he worries of Mewtwo's fate,
then, a gigantic shadow with a cape on it starts to land from above with energy waves coming from the ground, and then a figure wrapped in a brown cape appears and Pikachu looks up at it, the figure takes off it's cloak and is revealed to be Mewtwo, then Pikachu says a happy "PIKA!" before the cinematic ends.