i saw this so mega garchomp might already be ready for release (although it wont be released yet). this juanca is the same guy we got the mega lucario sprites from so i'd assume it's already been collected.
no sign of a mega mawile spritesheet though so do that first ^_^
I can't see this being particularly likely, as it would mean too great a proliferation, unless they were to be sold at inordinate prices as a currency sink. There's just too much cash in-game. Those mega stones that were obtained for free from loot balls were never intended to be available from such a source.
Not only that, but aside from legends and pseudo legends, I don't think there are any Pokemon capable of mega evolution that can't be found in the wild, now that many previously box-exclusive Pokemon can be caught in the safari zone.