I wouldn’t count on many older Mega Evolutions being likely to get Mega Evolutions. Three or four a (Japanese) set would seem most likely, and, well, if these cards are only going to have half a year’s run (Which perhaps they will), that may make only three or four Japanese sets worth, plus some promos and whatever comes in the holiday set. Kind of tight, even for the unreleased Megas.
Hopefully at least Mega Lucario is featured, not just because Lucario is popular or even because Ash’s could Mega Evolve, but because it was the first Mega Evolution players could use in the video games, giving it strong sentimental value to many (At least I would imagine).
By the way, I’m kind of counting on M EX’s shooting up in cost when new fans sink their fingers into ZA and begin to form memories of these fascinating forms.