They really found a way to have almost every Mega (that’s programmed in, anyway) appear in the game in some capacity, outside of the player’s own choice of use:
Absol, Camerupt, Victreebel, Slowbro, Hawlucha, Beedrill, Banette, Barbaracle, Mawile, Ampharos, Altaria, Froslass, Venusaur, Tyranitar, Starmie, and Dragonite are all Rogue Megas.
Charizard X, Gyarados, Malamar, Manectric, Clefable, Skarmory, Excadrill, Scolipede, Scrafty, Eelektross, Chandelure, Pyroar, Dragalge, Drampa, Falinks, Floette, and one of the Z-A starters you didn’t choose are all major characters’ aces.
Lucario is loaned by AZ to serve as the tutorial Mega once again.
Blastoise, Alakazam, Gengar, Kangaskhan, Aerodactyl, Heracross, Gardevoir, Gallade, Aggron, Garchomp, Pidgeot, Steelix, Pinsir, Salamence, and Metagross all appear during the climax during the gauntlet to get to Prism Tower. (You don’t actually battle Mega Pinsir, but I guess to make up for that we see a whole swarm of them chasing after Grisham and Griselle, and also see next paragraph.)
Pinsir, Houndoom, Medicham, Abomasnow, Sableye, Sharpedo, Glalie, Lopunny, and Audino can all be used by late/post-game Z-A Royale opponents. (As can Tyranitar, Aerodactyl, Pidgeot, and Gallade, who were also accounted for in the previous paragraph.)
And Mega Zygarde gets the big climactic story moment.
Thus the only Megas that don’t really get showcased are:
1. The starter that’s weak to yours (since Urbain/Taunie Mega Evolves the one that has the advantage over yours, though they own both)
2. Chesnaught, Delphox, and Greninja, because Game Freak chose limited-time distribution fuckery instead
3. Charizard Y, for some reason (but who knows, maybe Leon turns up in Hyperspace Lumiose in the DLC because he got lost on the way to the post office and Mega evolves his Charizard at you lol)
At any rate, we’ve come a long way from Megas being barely featured at all in XY, and only slightly more featured in ORAS.