If Allister appears he deserves a good plottt....and I mean one that surpasses the best Pokemon movie or episode
Maybe, but I doubt it, that's why the developers deliberately gave Ash a Pokemon Sword concert tour.
Here are the reasons:
1. Zacian - in the forest he chose him as the sword hero from Galar, in the fourth episode of Sword & Shield Arc, Ash has just picked up the sword.
2. Bea - they chose this person instead of Allister as if Ash chose the Pokemon Sword tour.
3. Pokedex about Dracovish in Pokemon Sword: Powerful legs and jaws made it the apex predator of its time. Its own overhunting of its prey was what drove it to extinction. (However, It wasn't Ash's pokedex talking about it, but Cara Liss mentioned it).
4. Pokedex about Sirfetch'd in Pokemon Sword: Only Farfetch’d that have survived many battles can attain this evolution. (Pokedex sounded the same in the episode where he fought Gallade).
Perhaps Allister will show up, but I doubt there will be a fight. It will probably be the same as in Unova, Ash was supposed to go to the dragon village but Iris did not recommend him, instead he chose the leader of the poison type gym, despite this he met Drayden anyway, but without the pokemon fight.