I can think of a few myself, but I'd love to hear what others think.
Porygon-Z: If not a Mega, then at least some kind of buff akin to the one Scolipede or Staraptor got this gen. A slightly raised SpAtt isn't worth a drop in defenses with that speed. Masquerain: Looks cool, but, imo, it would have been better to have stayed Bug/Water. Aside from being a cool type combination, it doesn't have a 4x rock weakness. We have enough Bug/Flying mons, imo. Xatu: As one of only three non-mega Magic Bounce'rs, I think this thing would really benefit from some kind of buff. I would especially love a big defensive buff. Kingdra: As the 4th Dragon to appear ever, I think this thing deserves more. Smeargle: A Mega Smeargle with Prankster would simultaneously be the greatest and worst thing ever. Gallade: Gardevoir got one. Besides, Fighting/Fairy ftw. Sableye: It did wonders for Mawile, why not Sableye? It has weaknesses now, so I see no reason not to. Glalie: Give us a reason to use this over Frosslass. Milotic: These things were always an absolute pain in the arse to get, so why not? Regigigas: Remove Slow Start. Replace it with any other ability, besides Truant or Defeatist. Stunfisk: Awesome typing, less than awesome possibilities. Bisharp: Great mon, one of my favourites, but the thing really needs more speed or better defenses.