For the Megas design I was going for a more bulky appearance for minum and more offensive appearance for plusle.
The ears on Plusle increase in length appearing offensively. Plusle grows sharp, long calls helping it bend
(reference) electric type moves. Mega Plusle will receive the ability Lightning Rod, which is also its Hidden ability
Mega Minum gains a more bulky appearance with the larger, flat eats and stubby arms. Minum would be the bulky of the two, gaining the ability volt asorb (once again its hidden ability)
Both megas already contain rolls both can perform, some better than the other.
Both Megas will get new moves: Aura Sphere, Physic, dark pulse, dazzling gleam.
Mega Plusles new stats: 60/80/50/115/75/115 Mega Plusle would be frail defensively making it less of a threat. Without focus sash the Set-Up version makes it easy to counter, another down side but mega plusle's offensive stats make up for that. Mega Plusle would be Electric/Fairy type
Some Calcs:
252+ SpA Plusle Psychic vs. 0 HP / 4 SpD Garbodor: 232-274 (77 - 91%) -- guaranteed 2HKO
252+ SpA Plusle Dark Pulse vs. 0 HP / 0 SpD Mismagius: 170-200 (65.1 - 76.6%) -- guaranteed 2HKO
Plusle Sets:
Offensive Plusle:
Plusle
Ability: Lightningrod
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
- Volt Switch
- Dazzling Gleam
- Grass Knot
- Dark Pulse
Set-up Plusle:
Plusle
Ability: Lightningrod
EVs: 252 HP / 252 Spe
Timid Nature
- Baton Pass
- Agility
- Charge Beam
- Volt Switch
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Mega Minums new stats: 60/70/90/85/115/95
With Mega Minum being a fairy type, it has another weakness, poison. Other bulky pokemon like Muk and Garbodoar could set up and do a lot of damage with toxic/gunk shot. This would cripple mega minum. Taunt would also be a problem. It would completely shut down the use mega Minun.
Calcs:
252+ Atk Garbodor Gunk Shot vs. 252 HP / 252 Def Plusle: 294-348 (90.7 - 107.4%) -- 43.8% chance to OHKO
4 Atk Sandslash Earthquake vs. 252 HP / 252 Def Plusle: 186-218 (57.4 - 67.2%) -- guaranteed 2HKO
252+ SpA Camerupt Lava Plume vs. 252 HP / 252 SpD Plusle: 90-106 (27.7 - 32.7%) -- guaranteed 4HKO
Minum can do the same roll but more bulky.
Set-up Minum
Minun
Ability: Volt Absorb
EVs: 252 HP / 4 SpA / 252 SpD
Calm Nature
- Wish
- Charge Beam
- Baton Pass
- Protect
Fully Defensive Minum:
Minun
Ability: Volt Absorb
EVs: 252 HP / 4 SpA / 252 SpD
Calm Nature
- Encore
- Wish
- Counter
- Volt Switch