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This concept has brewed in my head a while and is not fully formulated. But I'll share anyway. Long post.
It has been 18 years since Abilities came into the picture, and 10 for Hidden Abilities. The franchise has gotten BLOATED, now beginning to feel as though just 3 possible Abilities, or only 1 at a time, is not enough for many Pokémon anymore.
So it goes like this: Hidden Abilities are removed entirely—those now become part of a Pokémon's normal Ability pool. Instead, Abilities are now divided in two different flavors: called Key Abilities and Lock Abilities.
A KEY Ability is something a Pokémon ALWAYS has, ALWAYS. From womb-to-tomb, they will forever be guaranteed to have this Ability on them. And it is always that SAME Ability, one which all members of that particular species share.
Every Pokémon also has 1-3 choices for a LOCK Ability, which is something NOT immediately available to a Pokémon upon capture, instead being earned over the course of time. (Never worked out how.) But this does allow for every Pokémon to eventually have two Abilities simultaneously, once fully trained.
Before you freak about this being overpowered or complicated, A) there's still now only 4 Abilities a Pokémon is associated with, with 3 possible choices like before, so it's not much more complicated, and B) it will not be overpowered because each Key Ability is tailored to that species. You won't get Sheer Force+Iron First Conkeldurr, because you won't be given overpowered options like that by design.
A quick example
>Wishiwashi
>KA: Schooling
>UA: Swift Swim or Water Veil
>Altaria
>KA: Cloud Nine
>UA: Serene Grace or Cotton Down or Fluffy
Do you see what it does? In the first example it gives something previously FORCED to have only one Ability now have more options, but also making sure it isn't overpowered. In the second, it kinda felt like 3 Abilities wasn't enough before, and it assigns its Key Ability as a thematically fitting Ability that Altaria is known for having.
