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Give Ditto an evolution
Ditto has 288 base stats
His evolution would have 500 base stats. The 212 additional points wouldn't be pre-determined. It would randomly distributed among all stats. How the stats are distributed would be determined by item held, other pokemon in the party and location.
Ditto's evolution's hidden ability would be a new one called Permanent Transform. When it uses Transform for the first time, the change is permanent. It completely becomes the other pokemon forever. It gains a gender, it can no longer breed outside that pokemon's egg group, transforms his replaced by the first move this pokemon has, its ability changes to an ability of the other pokemon, it changes its type accordingly, it can now learn every move this pokemon can, and if it doesn't have a nickname then its name change to the specie of the other pokemon. However, it keeps its nature, stats, IVs and EVs.