I got this from someone else's comment on another forum but since we all know who Ash is gonna use for his full battle, how about Kukui?
We know he has 3 Pokemon but he needs 3 more obviously unless he smacks Ash's team with only Braviary, Lucario and Incineroar
-Lycanroc: Nope, I don't think there will be more dog battles.
-Snorlax: It has a wide movepool and could be an interesting challenge for Ash to overcome.
-Magnezone: Doesn't look too much like his style, but it would be an interesting foil to Melmetal.
-Ninetales: Doesn't look too much like his style, but Lillie's reaction would be priceless. But if they want to reference it they could change it for its counterpart Alolan Sandslash.
-Crabominable: Would be an interesting challenge as we've yet to see it battling, but if Hala uses it its chances of appearing will be very low.
Other options from the Alola Dex:
-Trevenant: A reference to SM125, obviously it would be a different one.
-Ledian, Flygon: They kinda fit with the superhero aesthetic.
-Alolan Muk: We haven't seen it much, it would be a nice reference to Ash's Muk.
-Arcanine: Another fan-favorite that could distinguish itself easily from Incineroar.
-Hawlucha, Primeape, Poliwrath, Mienshao, Machamp, Passimian, Scrafty: I think Lucario will be the only representative of the fighting type, but I'll put them here just in case.
-Heracross: I think it has a higher chance as it's not defined as a fighting type, and could be the counterpart to Guzma's Scizor.
-Salamence, Kommo-o, Dragonite, Metagross: As others have said, it's likely he will have a pseudo.
-Kabutops, Armaldo, Archeops, Carracosta, Tyrantrum: He is a researcher so it would make sense he would have a fossil, and they would great add type coverage.