>>22565701Mega Banette, Absol, Manectric, Mawile, and Sableye would all benefit from having their Megas as final evolutions, for reasons like being able to use items, allowing better Megas on the team, and having their ability activate immediately.
OP ones and ones that go beyond the third stager are ones that should be Megas.
(that being said, Megas should really be about providing a different setup and playstyle for the original poke, like M-Pinsir and M-Gyarados)