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You know, my interest in the anime has opened me up the idea of a mega Pikachu, granted before they mega it they need to deal with some inconsistencies.
-Extreme Speed is no longer an event only move and can be bred onto other Pikachu from the event one.
-Contest moves can be bred off of contest Pikachus.
Mega Pikachu would probably need an ability that works with the super speed theme the anime has going, considering it's the base Mega Pikachu would come from. Speed Boost would be good, if you want to go into special snowflake territory, Moxie for Speed instead of attack.
HP: 35
Attack: 95 (+40)
Defense: 40
Sp. Atk: 90 (+40)
Sp. Def: 50
Speed: 110 (+20)
Standard set doesn't seem to be really working, so let's try a one sided set.
HP: 35
Attack: 135 +80
Defense: 40
Sp. Atk: 50
Sp. Def: 50
Speed: 110 +20
This is better, allowing a set of Volt Tackle, ESpeed, Knock Off, Contest Move. But it leaves a lot to be desired specially.
HP: 35
Attack: 55
Defense: 40
Sp. Atk: 130 +80
Sp. Def: 50
Speed: 110 +20
This also works but is limited to Pikachu's special moveset. Thunderbolt, Volt Switch, HP, Grass Knot would probably be the best set.
Now if we ignore standard mega rules
HP: 35
Attack: 115 (+60)
Defense: 40
Sp. Atk: 110 (+60)
Sp. Def: 50
Speed: 120 (+30)
Much better, solid UU.
If Minmaxing is what we need.
HP: 35
Attack: 125 (+70)
Defense: 20 -20
Sp. Atk: 110 (+60)
Sp. Def: 20 -30
Speed: 120 (+30)
Not much more Powerful, but a lot more frail.
Make of it what you will.