Alexander18
Dragon Pokemon fan
Well since Kalos is almost over I might as well say what I wish to happen. Mewtwo from the first movie should come back and train with Ash. Not travel on a journey but whenever Ash has to fight someone like Tobias in a future Pokémon league Mewtwo will want to test its strength against these strong Pokémon. I also believe that it should evolve into Mega Mewtwo X to showcase the Mega Evolution. I know that many of you will say Mewtwo is too proud of an intelligent Pokémon to want to associate himself with Ash, however like I said, the reasoning could be that he wishes to test his strength. I know the writers don't like doing things like this because it's fanservice, but hey! One can dream right?!!
Edit:
Another reason it could train with him is because he believe the strong bond in Mega Evolution between trainer and Pokémon is the only way to increase his strength.
I seriously doubt Mewtwo would appear or train with Ash. With the Kalos league currently being held and with only several episodes left of the Kalos region, I don't think Mewtwo has any reason apart from X mega evolution to appear in the series. Ash is certainly not going to train Mewtwo in my opinion.