We don't see Ash in the games, so I highly doubt anyone in the new Pokémon anime series will be in the game unless they were designed specifically for the game.
This is a super interesting thought, thanks for sharing!
Marnie having a Morpeko, which is Electric/Dark, works really well for this (though I’m sure you’ve already noted that), as well as Team Yell using the Galarian Zigzagoon line, which is Dark/Normal, and being huge fans of hers. If she were to be the child of/future heir to the Galar Dark type Gym, and it’s never or very rarely promoted to the Major League because it’s Dark, that would be a really interesting story.
Though it might upset people if that means it can’t be battled.
If that's the case, I'd guess they'd be postgame content, as something special for you, the protagonist, for letting them have the credit they deserve. It is an interesting idea.
That being said, I'm guessing Team Yell uses the Galarian Zigzagoon line in part because they share Team Yell's loud aggressiveness and because, as their goal is to impede and obstruct people, Obstagoon is the perfect Pokémon for them.
Maybe the reason there haven't been other dark type gyms is because of how demonized they got by Rose? If Galar is where Gym Battles started then it stands to reason that they have an influence on what region has what gym. Marnie's family could've figured out that something shady was going on so Rose pretty much banished their gym to the minor leagues, they try to move and start their own gym in another region and Rose tells the other leagues that, "Hey, some shady people are trying to start a dark type gym as a cover for their criminal activities, we have reason to believe that they have connections to Giovanni. Close all dark type gyms until further notice." A little bribe here, some manipulating of the champions/E4 there, and Marnie's family have to stay in Galar. Heck, throw Grimsley into the mix, something made him leave the E4, maybe he's related to Marnie and the whole thing affected him to.
If so, would that mean that Alola truly does not operate by the same rules as anywhere else? Then again, I guess Nanu's a Kahuna, not a Gym Leader, so technically, he wouldn't be bound by those rules.
Or maybe it's just because most people don't want to be known as the "Evil" Gym.
Despite that, we do still have Dark-type Elite Four members (and the aforementioned Nanu, who's one of the biggest forces of good in Alola), none of whom are actually evil except for Malva. But I guess the Elite Four keep a lower profile than Gym Leaders. Alola is the only region I can think of where they don't just remain cooped up at the Pokémon League and you never see them anywhere else doing anything else. (You also get to see Grimsley outside of the Pokémon League, where he looks like he has a lot of pent-up energy he needs to release.)
