This final few days is honestly going to be some of the scariest for us, because we'll either get a reveal, or nothing, as per usual.
Even if we still continue to get nothing, up until the game is released in Japan, it won't mean he isn't in the game, it will just prove that he is unlockable and we should see that confirmed once the Japanese players unlock all the characters. Otherwise, if there is nothing of him still, there is always DLC (but I doubt that Mewtwo will be DLC - can't see it happening.)
So even if things do look grim, there will always still be two more chances.
I'm really hoping we see him because I've remained so optimistic about his return for pretty much the past 3 years since hearing about Sm4sh. I will be pretty crushed if Sakurai once again decides to let us down and not include him. I'm repeating myself now, but as I said before, it would not make sense for Sakurai not to include him in the game.
The roster is well finalized by now with less than 5 days to go. I really really hope we see Mewtwo.
I really do.