So this has to break rules to work, but:
Mega Ditto
Type: Normal
Ability: Perfect Cell (Ditto transforms into its foe, then gains a 30% boost to all stats except HP)
HP: 48
Atk: 48
Def: 48
SpA: 48
SpD: 48
Spe: 48
So the idea here is that a 100 point increase in anything other than HP wouldn't help Ditto (and allowing 148 HP would be broken), so instead, Mega Ditto gets to mega evolve into a superior copy of its target.
The ability "Perfect Cell" is very similar to Huge Power, but applies a 1.3 gain to all stats except HP. Therefore:
Mega Ditto retains its pre-mega base HP of 48
Meg