>>22737355A big meh on the first to be honest Steven wasn't really defining in Ash's path that Diantha is now. He's nearly Loreli tier on the list of Champions who have an impact on the main story.
And what makes you think Ash will get a Mega? I doubt it. Especially since he'll be going to a new region. The only two Pokemon who really qualify are Pikachu, who can't Mega Evo, and Charizard, who shows up probably the most, but still not enough to be a Mega Evo. Why on Earth do you think they are even doing the Mega Evo specials? If anything, Ash's philosophy is and always has been to overcome the limitations of those who are lesser evolved. And Pikachu feels the same way.
Mega Evolution isn't a throwaway thing. It can only be done with the Pokemon whose hearts and path are shared. Really, only Pikachu qualifies in that respect.