A friend has told me that Masuda plans to have, if not all evolved and those with no evolution, most Pokemon to receive Megas.
Here's my guess, some Mega forms might be in and out gradually. By 'out' I don't really mean chopping that Mega off the board, it's more like changing the rosters, akin to those of Fighting games, where some characters appear in one series, then disappear and returning back again? :D Just my two cent, basing off my friend's words on 'Masuda'a plans' and seeing how GF caters the competitive scene in Gen VI.