I think some fifth generation pokemon could have mega-evolutions ( since there aren't any who have them ) like Druddigon, Golurk, Haxorus... and I don't see why any other starter pokemon couldn't have a mega-evolution?
I agree with you here that they should include some fifth generation Pokémon, but I don't really agree with your choices. Druddidon, Golurk, and Haxorus are already big, hulking things and they kinda look "mega" already. Personally, I think Scrafty, Musharna, and Mienshao could use mega evolutions.
Additionally, I think that some sixth generation Pokémon should have had mega evolutions, like maybe Hawlucha and Heliolisk. It seems weird that, in the very generation that introduced the concept of Mega Evolution, none of the new Pokémon had Mega Evos.
On top of that, all the starters from each gen should be given Megas. Its like they're favoring some of the starters over others or saying that Charizard, Blastoise, Venasaur, and Blaziken are better than the rest. But, in my opinion, the whole appeal of the starters has been that they're on the same level.