I feel like there are more Pokémon that need a Mega Evolution. Mega Rapidash comes to mind. A Pegasus design with the Aerilate Ability would be great for Rapidash, which isn’t very powerful on its own. True, giving it a Flying typing would give it an even bigger weakness to Rock type moves, but if done right it could still be a powerful Mega.
Also Dragonite and Goodra are the only Pseudo-Legendaries without a Mega and none of the Sinnoh, Unova or Kalos Starters have a Mega either (even though Ash Greninja is just a Mega-Evolution without a Mega-Stone), whereas all of the Kanto, Johto and Hoenn ones do.