Well, i thought the only time he can get back into the anime is when they introduce the "champions league" where all champions from all region compete against each other.. I can't really see him become the villain, given that he himself isn't actually an antagonist or evil..
Us knowing about him beforehand wouldn't have changed much, though. Ash still would've lost. :c
I'd say in Champion's league.
So never? Because at this rate we'll never see that event since Ash can't even win a League.
Ash-Greninja for the rematch would be SOOO hype! Z-moves would be cool too
By reintroducing him as the next league finals opponent and once again serving as a troll.