Since Mega Lopunny got confirmed, I think I'll go over more ideas, including the mons that CAN be found in Hoenn that deserve Mega Evolutions, IMO.
Mega Golduck (Water/Psychic. Having this combo could be very useful if Mudkip was not your chosen Starter. Plus, if it got either Swift Swim or Adaptability, this guy could seek greater use in the competitive environment.)
Mega Xatu (Psychic/Flying. Give this guy Air Slash to utilise its' good Sp. Atk, retain Magic Bounce and we can call it a day with this guy.)
Mega Probopass (Rock/Steel. It should gain Filter and a considerable Sp. Atk boost to become something that you will not scoff at in battle. Only because it is the evolved form of Roxanne's ace Pokémon, Nosepass. Definitely a potential candidate on her team to Mega Evolve.)
Mega Raichu (Electric/Fighting. Raichu could learn a handful of Fighting moves throughout the past generations, and if some luckily come back as Tutor Moves, Mega Raichu could be quite a threat.)
Mega Dusknoir (Ghost/Dark. Only cause Banette had one and may not properly evolve again. Mega Dusknoir could be more offensive, sporting great Attack and Sp. Atk, which may cost it some Speed...)
Mega Cacturne (Grass/Dark. It would be really hilarious, yet very beneficial if this guy got Speed Boost. It should have improved defences to help it take hits a little better.)
Mega Walrien (Ice/Water. Having improved Defense and Sp. Def, maybe along with Sp. Atk and Attack can give this mighty walrus a possible spot in a slightly higher tier. It should definitely keep Ice Body, but Adaptability doesn't sound like a bad idea.)
Mega Crawdaunt (Water/Dark. Whatever way it may work, it definitely needs to keep Adaptability to keep making its' Aqua Jets pose a threat. Some better physical bulk and better speed will help this guy, even though he's awesome enough as he already is.)
Mega Hariyama (Fighting. It can gain some better defences to take a handful of hits like a boss. Thus, it would be logical if Brawly's ace Pokémon is the one that Mega Evolves in a potential rematch.)