Artificially made Pokémon.
What if the gimmick this gen is artificially made Pokémon based on ones that are extinct in a not too distant future (practically metal version of them aesthetically speaking, but with different move-sets and slightly different base-stats)
The plot would be saving the Pokémon world from a company/team which, because its activities, is seriously affecting Pokémon habitats putting them is endangerment.