If Cynthia is to appear in the Kalos region, then she will most likely have a Mega as well due to Diantha having one, so I was thinking that it's pretty likely that the other Champions would most likely also have Megas in the case that they appear, so I listed the possible ones from the Pokemon that they've been shown to use.

Lance: Gyarados, Aerodactyl, Garchomp, Kangashkan, Tyranitar
Most likely Aerodactyl if Wallace doesn't get Mega-Milotic in the future.
Possible Mega-Dragonite because it's his signature Pokemon, it's useable, though the fact that he uses multiples rather than have connection to a

Steven: Aggron

Wallace: Gyarados - Future Mega-Milotic possible because it's his ace and somewhat popular because of Feebas

Cynthia: Garchomp, Lucario - Most likely Garchomp unless GF decides to jerk Lucario off even more

Alder: N/A - Future Mega-Volcarona possible because nobody gives a shit about Bouffalant

Iris: Aggron - Future Mega-Haxorus possible to not conflict with Steven