A reliable source sent out some new leaks about MegaEvolutions of past Generations pokémons. Here a resume for you:

MegaWobbuffet > Gains the ability Counterstrike, which reflect 1/2 of the direct damage taken back to its source;
MegaLuvdisc > Becomes Water/Fairy and gains the ability Gorgeous: same sex pkmn have their Atk halved, while opposite sex pkmn will instantly fell in love; no effect on genderless pkmn;
MegaPorygonZ > Gains the ability Hacker, while he is in the battlefield no weaknes, resistance, or immunity applies;
MegaBastiodon > Gains Towershield, working as Reflect as far as he remains in battle;
MegaRampardos > Gains Scourge, and all his physical attacks hurt also pokémon adjacent to its target;
MegaMeganium > Gains Florescence, boosting some kind of moves like Aromatherapy (which gains an addictioal effect like Safeguard) and Leech Seed;
MegaTyphlosion > Becomes Fire/Ground and gains Aridity, which boosts Ground-type moves and INDEBOLISCE Grass-type moves;
MegaFeraligatr > Becomes Water/Steel and gains Strong Jaw, the ability that boosts bites;
MegaSceptile > Gains Suction Cups (Meh...)
MegaSwampert > Gains Poison Touch
MegaTorterra > Gains the ability Leviathan, which renders it immune to stats drops (both from enemy moves and its own moves);
MegaInfernape > Gains Stagger Rage, which makes it immune to confusion and gives all his physical moves a 10% chance to confound the target;
MegaEmpoleon > Gains Regality, everytime he defeats a pokémon one random stat is boosted by two stages;
MegaSerperior > Becomes Grass/Dragon, keeps Contrary as his ability (Serperior with Overgrow change ability to Contrary when in Mega form);
MegaEmboar > Gains Berserker, its Attack and Speed are boosted at max stage when under 1/4 HP;
MegaSamurott > Gains Bushido, becoming immune to effects like confusion, attraction, taunt, provoke, etc.

These the informations my source gave me, it's a reliable source who predicted other news from XY