I wonder what system will the battles be held.
If it's a tournament with quarter-finals, semi-finals and the final, then I guess Ash's opponents are obvious - Iris, Alain and Leon, probably in this order. The rest seem to be just "background NPCs" and they seem to be there mostly for flavor.
I don't count on Full Battles, zoomers would probably get too bored. So, I'm guessing 3-on-3, like the Raihan battle.
Doubt we'll see much of the non-Ash battles, probably just short glimpses of one Trainer's ace beating the other's ace. Though, I wish they do show some previously-unseen Pokemon for Trainers such as Lance, Steven or Diantha (if not in those short scenes, then at least maybe displayed on the screens that show their Pokemon - I like it when they acknowledge that the Trainers have other Pokemon aside from their aces). If the battles are 3-on-3 then Leon is guaranteed to have one more Pokemon (probably Aegislash, yaaaaawn) and I guess they may give something new to Alain, since his Pokemon other than Charizard weren't especially memorable. I don't expect any surprises in Dawn's team - probably Haxorus, Dragonite and Excadrill.