With so many Hoenn Pokémon getting their own mega-evolutions, I'd like to see Metagross get one as well. Appearance-wise, maybe it could be darker in color, have heavier arms and legs, and smaller eyes. Due to the addition to its weight, its defensive stats could be increased a little. Its Speed would be lowered, though. And since its eyes are smaller now, perhaps that could help it see better and give a little boost to its accuracy. d:
I would also like to see them give us a mega-evo form to Skarmory. Mega Skarmory could have a raise in its Attack, Special Attack, and Speed, maybe, and have its HP and defensive stats decreased a bit. When it comes to battling, Skarmory are usually used for their defenses. But Mega Skarmory, though, would be used more for the offensive side of things.
Other Pokémon that I'm hoping to see a mega-evolution for is Swampert and Sceptile. Blaziken has a mega form, so why not the other two? Blaziken, Swampert, and Sceptile are equal, so they should be treated as such. I could imagine Mega Swampert having a larger body, which would enable it to stand more attacks and stall your opponents longer. (: As for Mega Sceptile, maybe they could make it look like some sort of a dragon?