I will try to describe what Mega Stone I can see with Ash and the ones that I don't see Ash with them.
X/Y
Mega Venusaur: i don't expect it.
Mega Charizard X: Yes, because it was very very popular. Even though Alain has already a Mega Stone.
Mega charizard Y: Yes, because it has yet to debut and Ash's charizard is one fo the most uitable pokemon to Mega Evolve
Mega Blastoise: I don't expect it.
Mega Alakazam: Not expecting it because requires a trade
Mega Gengar: Very unlikely because of trade, although it's apparently popular.
Mega Kangaskhan. Fortunately it isn't appear to be very popular. And thanks because I don't want to see it in Ash's hands.
Mega Pinsir: Very unlikely.
Mega Gyarados. I don't see it because Lysandre.
mega Aerodactyl: I don't see it because it's obtained from a fossil.
Mega Mewtwo X/Y: NO.
Mega Ampharos: Yes, but it's not the most likely option.
Mega Scizor: Yes, it's relatively popular enough that I see in Ash'ps team. But not the most likely option.
Mega Heracross: Ash can return heracross but it isn't the likiest option .
Mega Houndoom: Relatively unlikely, but not impossible.
Mega yranitar: I don't see Ash with that thing.
Mega Gardevoir: I don't see itbecause Diantha. However, is apparently very popular.
Mega Mawile: One of the most likely options for Ash.
Mega Aggron. It's possile, ut it's not he most likely.
Mega Medicham: Yes, but not the most likely.
Mega Manectric: It's possible but I think the possibilities are going to be very low.
mega Banette: I doubt it.
Mega Absol: My 1st option with difference it's not shared by ayone but Ayaka (and i think that Ayaka wasn't meant to appear again, because of other thing it was voiced by Miyu Sawai). It's apparently one of the most popular Mega Evolutions.
Mega Garchomp: It's one of the most popular megas but I don't see it because I think that Cynthia will return with this Mega.
Mega Lucario it's one fo the most popular but I don't see it because Korrina.
Mega Abomasnow: I don't see ash with it.
OR/AS Megas:
Sceptile: Yes, of course.
Blaziken: I don't see it because Limone.
Swampert: I don't see it.
Sableye: Possible but not likely.
Metagross: I don't see it. because Steven has one that is likely to be showcashed in the anime.
Lati@s: No.
Diancie: NO.