(Should be) Locks:
Ash:
Duh
Iris:
leaving out iris is pretty silly from a narrative perspective. She's already better than cynthia pending a z move, mega evolution, or gigamax feature and she's less than half the age of many of the proposed options(indicating potential). All you need for a rival for ash is there
Should be there:
Paul: In terms of what's seen he's the most skilled battler in the series and you can flip ash and paul's dynamic here. He's a super focused battler so...
Sabrina(kanto): No reason she has to be here, but she'd offer a much stronger potential narrative than anyone else and her being this strong makes complete sense
Bea: She was already very close to the masters tournament, is half the age of many current contestants(potential), is potrayed as competent and battle focused, and has some history with ash
Cynthia: Her not making it undermines her not retiring, also gives someone for iris to beat/have a rivalry with. (Iris should probably win while keeping her mons at base to show progress)
Good Enough(but):
Leon:
Considering he's already climbed the mountain top maybe he wants to retire, making room for a more interesting character
edit: just saw that ash loses in this hypothetical so you probably should keep leon there(i'd have him lose to cynthia to justify her non-retirement with a payoff)
Could be there:
-> Gary: should be skilled/strong enough and is easily ash's strongest rival narratively. Oppurtunity to flip dynamic
-> Kukui: rare example of a solid character from the anime with history and should be skilled/strong enough
-> Guzma: Guzma beating kuki to win a league would be strong narrative progression, could make a solid antagnoist
-> Corrina: I mean she has history with ash
-> Misty: sure, why not
-> Tobias: him getting swatted would be funny
-> Alain: **** character but has history with ash
-> Conway: definitely skilled enough, could see progression here in other areas
-> James: sure, why not
-> Blue hair from alola: offers a subtle character dynamic missing from the show, competent, focused