I see no reason to think that we aren't getting the bulk of XY megastones available normally.
Only 9 were available in the main story of XY, so there's a not insignificant chance that some people just never experienced the bulk of them.
Furthermore, all of the third gen megas (bar Blaziken for obvious reasons) were post-game. Not having those at minimum would be crazy town
And then looking at the first & second gen megas, we have Alakazam, Gyarados, Pinsir & Heracross. Again, perfect for putting in the main game now.