I like megas and all but I don't like how GF adds megas to some pokemon. Like Sableye or Mawile. Those Pokemon were underpowered Pokemon without an evolution, they should have got a normal evolution instead of a mega imo. Like Misdreavus or Murkrow. Slapping mega's on all those underpowered one-stage Pokemon seems a little too exaggerated to me.
That said, I'd really love to see a Flygon mega, a Milotic mega would be cool too, since Gyarados got one and they are kind of opposites to each other.
I see where you're coming from, but Game Freak's using Mega evolution as a way to make those one-stage Pokemon more useful, like a true evolution would have. Sure, it's a lot less practical, because then the Pokemon can't use held items, and it can be the only Mega on a team, but these draw backs allow for bigger gains in terms of stat boosts/ability changes. Not sure how it's exaggerated, as you said, but if that makes them under-powered, then wouldn't giving 3 stage Pokemon like Flygon a Mega over-powered? Point being, Megas have lots of different purposes. All Mega-evolved Pokemon are powerful and dangerous, doesn't matter how large their evolution line is.