I wonder if a possible solution to this is to say "you can use either a Pokémon that you have in a Gen 7 game, or a Pokémon that you have in a Gen 8 game". Thus, it's not a "pet mod" per say, it's just everything that Game Freak designed to do.
For example, you can have a Gengar that is holding a Gengarite. Or you could have a Gengar that knows Nasty Plot and Encore. Both of these are decisions that Game Freak made to allow you to do. But what you CANNOT do is have a Gengar holding a Gengarite that knows Nasty Plot and Encore. That's something that Game Freak never intended to exist, and never meant for you to have, or for it to affect the meta in any way.
What do you think of this proposal?
For example, you can have a Gengar that is holding a Gengarite. Or you could have a Gengar that knows Nasty Plot and Encore. Both of these are decisions that Game Freak made to allow you to do. But what you CANNOT do is have a Gengar holding a Gengarite that knows Nasty Plot and Encore. That's something that Game Freak never intended to exist, and never meant for you to have, or for it to affect the meta in any way.
What do you think of this proposal?