So, Alain and Ash will probably meet each other after Anistar gym match, but Anistar i remember have a thing correlated with megaevolution... the Sundial, in the games it allows to find megastones... maybe even (with some differences in comparison to the games of course) in the anime? All with Sycamore's explanation, of course, maybe ash and company will meet the professor here. And for the searching of megastones Sycamore could give a keystone to ash , maybe... (i dont known, if there is a megastone nearby the keystone could shine, etc) this make sense because Ash is traveling far and wide the Kalos region, so he have more possibilities to find a megastone.
After all Ash main problem with mega-stuff is finding a megastone, for the keystone there is Sycamore or Meyer (or others, maybe only a borrow, etc), and Ash already have many potential pokemon, one in particolar... and there is an empty space in the team.
Maybe in the middle searching of megastone, Ash will meet Alain (who is also searching megastones, like in Act I?). And maybe the megastone it's a charizardite Y ... Ok, i'm a bit too far here with my fantasies... LOL
It's only my possible teory, of couse, things could be very/totally different, but maybe something like this (more or less) could happen.