Gym Leaders:
Kanto:
Forrest- While its not Brock, I feel like this would be the next best thing, so Ash can have a decent battle with a rock type trainer (and by that I mean around 8th gym badge level rather than the 1st gym level).
Lt. Surge- Only because it would be interesting to see what a rematch would look like. Perhaps have it be a 3 on 3 with Ash winning again, to affirm that him winning the last time wasn't a fluke
Sabrina- Only because it would be nice to see the Haunter from before (as a Gengar), if they completely ignore it that's fine, a proper battle with Sabrina at least, please.
Erika- Proper battle like the others, though I won't be too broken up if it never happens.
Why not Misty? Sun and Moon was perfect for Ash beating Misty with Pikachu against her Mega Gyarados they don't need to battle again, though I wouldn't oppose it, but only if Pikachu loses to Mega Gyarados this time.
Giovanni- I would really like to see Ash battling Giovanni, I can imagine him actually being the strongest gym leader of all regions even including Galar with the obvious exception of Raihan (unless of course the Galar gym leaders are viewed as Elite Four level).
Janine- I just want Ash to fight Janine. This the one I want the most, because it would actually introduce her into the anime.
Johto:
Jasmine- Would be nice to see her with potentially new steel types to battle against Ash
Chuck- Bayleef bodied his Pokemon so I would like to see Ash struggling against him
Clair- A rematch with Clair would be nice.
Pryce- The last battle ended in Pryce forfeiting, I didn't really like that they just didn't want Piloswine to lose, this time I want a clearer loss.
Hoenn:
Wattson- Ash killed him in their first battle, and Manectric killed Torkoal in their 2nd match, I would like Wattson and Ash to have a competent battle where Ash actually wins. It would be especially interesting plot point if Wattson and Lt. Surge know each other.
Flannery- I could just say simply because....Flannery but I kind of would like to see her battle as a MUCH better gym leader
Tate and Liza- Not necessarily part of PWC, but I would like to see a rematch, with this time not using questionable anime tactics. Or maybe Tate & Liza could be an exception to PWC in that they qualify as ONE rank just to spice things up for challengers.
Winona?- Shiny Swellow?
Sinnoh:
Gardenia- Would be interesting just to see a rematch between them, and on top of that get James involved and we can either see Cacnea or Cacturne again
Crasher Wake- Ash killed Crasher Wake the first time, I want at least a 2 vs 2 match where Crasher Wake gets justice by knocking out ONE of Ash's Pokemon.
Volkner- This time I would like to see Volkner go all out. Yes that would include.......Raichu and with Lt. Surge that might seem redundant to have a third Raichu battle or even a 4th, I'd still like Volkner to be near Elite Four level and challenge Ash.
Unova:
Drayden- Because Ash never battle him before
Marlon- Because it wasn't Ash who battled him last time.
Cheren- A proper battle with Ash winning.
Kalos:
Korrina- Already confirmed but to add to this. Korrina truthfully is the only Kalos gym leader I want to see Ash battling. Only because I feel satisfied with how XY and XY&Z went that I really don't need to see the other trainers.
Alola: No gym leaders exist, and even if I qualified Kahunas, I'll just save those for general region characters
Galar- I want to say all 12 of them (yes including trainers like Bede and Marnie and yes qualifying them as Gym Leaders) my only issue of course I doubt Ash will battle all of them thus I'll list the ones I want the most: Nessa, Kabu, Allister (over Bea), Melony, Piers and Raihan but even then that's too much probably
Rivals: I'll just list rivals over the region for this section
Ritchie (over Gary): While I would like Ash to battle Gary this season, I don't want it to be as a PWC challenger. I want a rematch with Ritchie so Ash wins.
Casey: Despite doing gyms, Casey was replaced with Macey at the Silver Conference........for some reason, so I would like Casey to come back and have a true battle, maybe a culmination of all she learned and being able to put Ash in a corner, and hey maybe Ash will remember Beedrill.
Paul: I would like these two to duke it out at their full power and at their best. I fully expect Paul to have some 5 or 6 generation Pokemon at his side. Ash will win of course but it would be nice to see Paul acknowledging Ash as a really great trainer a lot more than the end of Sinnoh. I fully expect Paul to tease Ash about Tobias though.
Tobias- Full battle rematch, Ash uses his old Pokemon with some of his new Pokemon (like Dragonite/Gengar) and Ash thwomps Tobias into the ground and Tobias screams and cries about how he's finally been knocked down a pin. LOL, mostly kidding about that, but Ash beating Tobias would be neat.
Bianca- BECAUSE ASH DESERVED A PROPER BATTLE WITH HER. That is all.
Sawyer- I'm curious to see how a rematch would fair between these two. What new Pokemon would Sawyer have in comparison to Ash's new team.
Alain- If Alain comes back as a PWC opponent, I'm fine with Ash-Greninja showing up. It doesn't need to be a full battle, a 3 vs 3 battle would just be fine with Ash finally defeating Alain. To make it clear, I want this to happen in the Kalos region, and Greninja senses Ash and joins Ash for this battle, depending on Sobble's level, I could see some really cool dynamic between the two. Whether Greninja stays with Ash to rest at Sakuragi lab until certain moments I don't care, but I don't think its right to have a battle with Alain again without Ash-Greninja finally defeating Alain's Mega Charizard Y (LOL).
Alola?- I don't see anyone in Alola coming back to be a PWC opponent that qualifies as a rival necessarily. It would either be Gladion or Kiawe, but I just doubt they would be PWC opponents, I think its too soon for them, Gladion because his father and Kiawe because I can't help but think he's too busy training under Olivia.
Anyone I missed? I don't really care about them, but maybe I forgot someone but assume I don't care about them
Elite Four- While I could say I would like Ash to battle all the Elite Four member let's be realistic......not happening, instead I'll choose my favorites (again I'll list characters over regions for this)
Agatha- Only because I would really like to see Ash's Gengar beat Agatha's Gengar, if not then Pikachu can win in the rematch between the two.
Lorelei- To make up for the stupid Prima dub thing
Karen (over Will)- I think it would be interesting for Ash to fight a really tough Dark type trainer, would be probably useful for Lucario and potentially Inteleon
Phoebe- I know I know, I'd just would like Phoebe to appear.
Drake- Because Ash battled him before and lost, how about a proper conclusion with Ash winning.
Bertha- Rematch though I honestly have hard time believing Bertha would care about the PWC. Would be nice to see Bertha acknowledge as a true potential trainer rather than mocking him for only having 7 gym badges (despite Ash actually having MORE than that).
Flint- Rematch purposes
Caitlin- Because she already showed up. Don't get me wrong, I wanted Shauntel and Grimsley but GEEZ we already have enough type coverage, plus this makes up not choosing Will or Lucian.
Malva & Siebold- While I have nothing wrong with Wilkstrom or Drasna, and I would be okay with Wilkstrom showing up, but to be fair, Malva and Siebold battling Ash would be more exciting in my opinion, if only because they are already canon, while we never saw the other two. I feel like in this instance (and only this instance) canon over implied canon. Though Wilkstrom did show up in a movie though.
Champions- Choosing my favorites for this, it's hard to say if Ash will battle all of the champions even as a practice battle (like Diantha/Alder)
Cynthia- For obvious reasons, if it's not obvious then to explain its because she has the most connection to Ash OVER all other champions, and while it will be hard to envision Ash beating Cynthia, when she curbstomped Paul, I'd still like Ash to battle Cynthia.
Steven- Only because he was redeemed as a character in the XY&Z arc, and got a better role. It's a shame he doesn't remember Ash or it was overlooked back then, but at the least he should remember the events of the Kalos region.
Lance- To see if Lance remembers Ash and to actually have a battle with him
Leon & Raihan- Only because it's likely for them but not my favorites.
Random Characters- Characters I could envision being in the PWC and battling Ash while not properly qualifying other sections:
Silver- Make Silver canon, and have him with some kind of dynamic with the Team Rocket Trio, while also butting heads with his father Giovanni. Maybe Ash beating Silver will spurn Giovanni from getting revenge, which could lead into an Ash vs Giovanni fight, with Ash winning, it also might make it so Team Rocket trio are ordered to respect Ash and not to try to take his Pokemon for example, Though I'm sure this would fit a finale series (Ash ultimately being retired), but it would still be interesting.
Jimmy- While I would much prefer Ash's Quilava coming back and evolving and being the first MAIN Typhlosion to appear in the anime, I'd still like to see Ash battling Jimmy. And as much as I would like Marina, we'll stick with Jimmy for this.
Lyra- I would like to see Ash battling Lyra and having a serious battle with her. That would be cool.
Iris- As much as I dislike Iris I don't hate her enough to not want her to come back (she's still regarded as Ash's friend and companion, so I can't just pretend like that doesn't exist). I would like to see Iris's Dragonite reacting to Ash's Dragonite. Hopefully Iris comes back with a much less condescending attitude. By the way I do NOT want her with Garchomp.
Alexa- I want Alexa to have joined the PWC as part of her journalism career, and I want her to have excelled, only to have her be stopped by Ash as they focus on Alexa traveling with Ash. Maybe even Alexa finding out about Ash being the Alolan Champion and wanting to battle him at his fullest. Maybe Ash can bring back Noivern for this battle, since I hate that the writers neglected to bring that up during XY&Z, I would've loved for Alexa to meet Ash's Noivern. Also Ash beating Alexa could mean Alexa exits out of the PWC and writes her article on the subject, just figuring that's how she could join the PWC.
Professor Oak- As an absolute surprise to everyone, Professor Oak is ranked #13 and Oak shows off near Champion level strength. Oak explains its been awhile since he let loose, as an even bigger surprise, Oak uses Ash's strongest Oaked Pokemon. Ash loses of course though Pikachu did take out three of the powerful but weakned Pokemon, Ash quits his dream of becoming a Pokemon master, he and Pikachu set up an exotic flavor ice cream truck, and of course because of Pikachu's insistence, Ketchup flavored ice cream, and their fate is never revealed to the audience as Professor Oak says "Oh well" while shrugging his shoulders. MOSTLY joking aside, it would still be cool if Professor Oak secretly was a strong PWC contender. bonus points if Oak helped found the PWC; April 10th, 2020, oh that's fine ignore this date, it's not important
Acerola- Come on now, we need an Ash vs Acerola fight
Kahlil- I would like to see Ash battling her would be nice.
Hapu- The only Kahuna I would like to see Ash have a rematch with, to be fair Hala would be a good choice, but I'm still having an issue seeing any Alola trainers being part of PWC outside of Acerola only so she and Ash can battle.