um... I will correct you on this. there is a special button to transform that can only be used while you are holding the mega stone that your pokemon needs not "trigger at the beginning of every battle if you are holding an specific items"
check your facts
My take on things: the mega stone is a smash ball and once you grab the smash ball you can use the special button to transform into mega mewtwo and the transformation will last until ether 1 minute has passed or you KO your opponent
that is a bad idea but hey I tried
(and I feel like an idiot for getting the names mixed up back there)
Sorry, I only saw that video today. Up until that demo, those were all the facts available.
On topic, I think Mewtwo's Final Smash could be much more interesting or eye catching than a Super Sonic style Final Smash. While I could see Sakurai just making him like a cross between Super Sonic and Giga Bowser, Mega Mewtwo currently lacks the pizazz both Super Sonic and Giga Bowser have.
Super Sonic is something that other players dread and the Sonic players love because he is stupidly overpowered and turns into a beacon of light, just like in the games.
Giga Bowser is a towering monstrosity that takes up the entire stages, becoming the focus of the battle and can not be ignored.
Mega Mewtwo would be at least as small as sonic and less flashy. He would also not be as imposing and eye catching as Giga Bowser, let alone being able to turn the battle around him.
While Mega Mewtwo has more power and speed than normal Mewtwo, he also shrinks in size considerably. I can more see his Mega form as being a special transformation that trades survivability for power and speed, than a proper final smash. A high risk, high reward mode.
I would much prefer some overpowering psychic attack that seems to take over both the field and the other character's minds, like an overblown Psystrike, for his final smash.
Who knows, maybe Mega Mewtwo will end up being more of a demigod of psychic power in the games/movie then I am anticipating. However, from what I have seen so far, the power up doesn't seem so extreme as to make it Final Smash level.