Here are my ideas:
- Mega Sandslash has the Grindstone ability, which raises its attack by one stage whenever it successfully uses a cutting or slashing move.
- Mega Ninetales is Fire/Ghost type. It has the Dazzle ability, which lowers the opposing Pokémon’s special attack by one stage (basically Intimidate for special attack). Flamethrower becomes Fox Fire, which is the same as Flamethrower, except it is both Fire and Ghost type (like how Hawlucha’s Flying Press is both Fighting and Flying type) and it may also confuse the target. For aesthetic appeal, the Fox Fire move icon blazes with purplish-black flames.
- Mega Arcanine is Fire/Dragon type and has the Flame Body ability. Extreme Speed becomes Mystic Speed, which is a Dragon type version of Extreme Speed. The Mystic Speed move icon blazes with deep green flames.
- Mega Marowak is Ground/Ghost type and has the Bone Lore ability, which increases the power of Bone Club and Bonemerang, and guarantees Bone Rush to hit 5 times.
- Mega Lapras has significantly boosted defense and special attack. It has the Drizzle ability.
- Mega Dragonite has some cosmetic features that reflect Dratini and Dragonair, while retaining the key features that make it recognizable as Dragonite. It has the Antifreeze ability, giving it immunity to Ice-type moves (I've stuck with this idea for a long time, but lately I've been thinking that it might be too OP. What do you think?).
- Mega Flygon has the Tinted Lens ability. Earthquake becomes Dragonquake, which is the same as Earthquake, except it is both Ground and Dragon type and it has a 10% chance of paralyzing those who are hit. The Dragonquake move icon has a scaly texture and it blazes with deep green flames.
- Mega Milotic is Water/Fairy type and has the Multiscale ability. Recover becomes Mystic Healing, which is Fairy-type and has the same effect as Recover, except it also randomly raises defense or special defense by one stage. The Mystic Healing move icon sparkles.
- Mega Tropius has the Solar Shield ability, which raises defensive stats in sunlight.
- Mega Bibarel has the Workhorse ability, which increases the power of HM moves.
- Mega Staraptor is Fighting/Flying type, thus getting a STAB with Close Combat. It has the Moxie ability.
- Mega Vespiquen has the Honey Feed ability, which restores a small amount of HP every turn as though the Pokémon were holding Leftovers. Its defense and special defense are drastically boosted.
- Mega Luxray is Electric/Dark type and has the Strong Jaw ability. Crunch becomes Volt Crunch, which is the same as Crunch, except it is both Dark and Electric type and it has a 10% chance of causing paralysis. For aesthetic appeal, the Volt Crunch move icon crackles with dark purple electricity.
- Mega Chatot has the Sonic Mastery ability, which increases the power and accuracy of its sound-based moves and makes Chatot immune to such moves.