i2i
Big Bad Wolf
And even then I'm not convinced that Ash would definitely showcase a Mega Evolution. It's like 50/50 at this point in my opinion.
I'm sure since Mega Evolution was first reveal that Ash was going showcase one even if he didn't have any Pokemon that were't capable of doing so. Because I believe that there is no way they won't have the main character of the anime not promote such big part of the current generation of games.