My idea is for an Gimmick called Extinct Form Pokémon these are Pokémon that change form and can change types, but they don't have to.
These Pokémon gain a fifth attack (Extinction Move), and a second ability (Extinction Ability).
These Pokémon also change their stats.
The Pokémon change into their Extinct Form once they would faint from damage. The Extinct transformation will nullify the damage they would've taken, and then changes them to their Extinct Form (So if they had 50 HP, and then would faint from an attack/weather/status, they will change to their Extinct Form, and still have 50 HP)
The lore is that the Pokémons' ancestors had achieved this form to survive an Extinction Event (like what killed the Fossil Pokémon), and they passed it on to their ancestors who are now capable of using it thanks to our destruction of the environment.
