One example of a Pokemon I wanna see get a Mega Evolution would be Raticate. Rattata and Raticate have always been somewhat of a nuisance, but to give Raticate a Mega would be awesome, especially if they made it bigger and more menacing and maybe even changed it's type. It knows a lot of Dark-type moves, so having its Mega be a Dark-type would make sense to me (I would say Normal and Dark, but the 4x weakness to Fighting-types would be a bummer if that were the case).
Just would like to mention that although it is outclassed by other Normal type Guts users in NU, Raticate does have a place in the competitive metagame where it can shine brightly, and if used correctly, has the potential into a force to be reckoned with. It would be cool to see a Dark type Mega for the critter, though. Would probably boost its usefulness a lot more than its Sucker Punch niche currently does. Would also make its main priority attack STAB!