These will be purely for fun! The most exagered sets i could made up, cause these mons are really terrible.
Pokemon Name: Pikachu
Mega Stone Name: Pikashit Pikachite
Stats: 35 HP / 10 Atk / 150 Def / 70 SpA / 150 SpD / 5 Spe
Typing: Electric/Flying
Ability: Thick Fat
New Moves: Floaty Fall, Tail Glow, Cosmic Power, Moonlight, Parabolic Charge,
Competitive Description: I bet you never imagine this! The defensive Pikachu! It earns flying because its magnetic field become so strong and it is so round that it became a probe balloon! which comes its space like abilities! Now you have a very good wall, its tail can receive signals from moon and use moonlight to give that fat ass some HP back, also it can get some cosmic power to raise defenses, since the moon controls the sea waves, it can use same technique for surf away oponents that resist electric. Now combine Tail Glow + Parabolic Charge you can recover almost all hp every turn.
Pokemon Name: Eevee-Mega
Mega Stone Name: Eevite
Stats: 55 HP / 55 Atk / 90 Def / 90 SpA / 100 SpD / 35 Spe
Typing: Fairy
Ability: Fur Coat
New Moves: Calm Mind, Recover, Sparkly Swirl, Sappy Seed, Cotton Guard, Bounce bubble
Competitive Description: Kneel before the best tank of the game! Spotting a enormous defense: stored power + cotton guard and calm mind for destroy oponents, sappy seed for increase longetivity. Cotton guard + recover + fur coat for "theres no chance it even be scratched", it will plays almost like lets go eevee! This is made for be the defensive mon in the game, only hold back by that creep hp, but it have all the utility neessary for be almost immortal, it suffers the most from the 4 slot syndrome, since without sparkly swirl/rest it can recover from toxic. But a Blissey with heal bell makes a great partner, oh boy this partners together will be the most annoying things to fight agaisnt. Hail Satall!
Pokemon Name: Meowth-Mega
Mega Stone Name: Meowthite
Stats: 40 HP / 120 Atk / 35 Def / 40 SpA / 40 SpD / 115 Spe
Typing: Normal
Ability: Protean
New Moves: Coil, Swords Dance, Extreme Speed, Close Combat, Ice fang
Competitive Description: Mega Beedrill on steroids. Since long meowth is flexible, it gains libero Protean dealing massive damage, it has a usable defense that can be boosted with coil (well i think it can coil with that body). Also u-turns from this thing will hurt as much as mega beedrill, but now with acess to stab priority extreme speed, stab close combat, even fake out to break sashes, it gets even ice fang to get a nasty coverage, imagine what it can do after just one swords dance? This will be the prime cleaner and sweeper, even good as ash greninja and kartana, althrough it cant switch in almost anything.