With this game, then can reveal new megaevolutions as they go along as all the game data can't be hacked like past games. So there could be tonnes more megaevolutions installed into the game but they haven't revealed them or given access to their stones yet.
If the Torchic event hadn't been distributed and MegaBlaziken offically revealed, there'd be no way to know that it existed since the stone isn't available in game. I feel like they've done that with more pokemon too.
Personally I would like Mega... Kabutops, Zoroark, Fearow and Cacturne.
Kabutops - gains Sap Sipper to make it immune to grass type moves. looks taller and spikier on it's back.
Zoroark - gains a different ability than illusion, which is too gimmiky for my taste. Maybe huge power. Looks more werewolf like.
Fearow - changes to flying/dark type. Huge attack increase. New ability. Wingspan is way broader with spiky feathers, colour is now darker with black markings, beak is black and sharper, big black claws. Bring it up to parr with other flyers (pidgeot, staraptor etc.).
Cacturne - Huge speed and attack increase. New ability that raises speed. Looks taller and slimmer and spikier.