I'm pretty happy with the Masters 8 roster as is. These are the things I'd wish for most if I had my way.
1) Ash's reserves splitting time with his JN team. They've clearly been training in their time away from him, and I'm sure they'd be eager to fight for him in the biggest tournament he's ever participated in and prove their newfound strength. In addition, these are champion level trainers who have been with their pokemon a long time, while Ash's team except Pikachu is still green in terms of how long they've been with him. It would only make sense to have some of his reserves join the fight. Of course, I wish they participated in the earlier matches on the way to the Masters 8 too, but oh well.
2) 6v6 the whole way through. This is related to the first. His JN team would still have plenty of time to shine, while still allowing (some of) Ash's reserves to directly contribute.
3) More dynamic battles. More opportunity for these trainers to show why they're in this tournament, as well as just more visually appealing battles. More dodging, more maneuvering, etc.
4) More interaction between the participants. We need to have met them beforehand for best effect, preferably along the way up the ranks, but at least before the Masters 8 began properly. Also, the losers from the first round not leaving and sticking around, so we get more interactions.
5) My biggest wish: double elimination. This is the scenario I would most want for this (*activating fanfiction mode*):
- Ash beats Steven to advance to the semi-finals against Cynthia.
- Cynthia beats Ash, her Mega Garchomp dismantling his planned strategy and mopping up his last 2-3 pokemon, the last one being Mega Lucario.
- Ash drops to the loser's bracket, and experiences a brief meltdown as he reflects on how handily Cynthia destroyed him once she had adapted to his strategy, and it hits him how intense the level of competition is at this tournament. He's been so focused on getting to battle Leon that he's forgotten that every other participant here is also among the strongest in the world, and he needs to slow down and climb the obstacles one by one. He gets his head back on straight and prepares for his next match with renewed focus.
- Ash's next match would be either against Steven, Iris, Lance, or Alain. Of the 4, I would prefer Alain, as he never got his chance to beat him. Would really like for Greninja to be the last pokemon here, for the revenge match against Alain's Charizard. Ash finally defeats him. Alain gracefully accepts defeat and tells Ash to keep his momentum.
- Ash's next match would be either Diantha (dropped to the loser's bracket after a semi-final loss to Leon), or one of the other 3 (depending on how the brackets go). Of these, I'd prefer Diantha. Ash advances after another hard fought match (maybe Dynamax Gengar gets to shine here?).
- The loser's final match is one Ash has been yearning for: a revenge match against Cynthia, who lost to Leon in the winner's final. He defeats Cynthia after a tough match where both participants have given it their all. Both trainers find their strategies getting thwarted throughout the match as they rapidly adapt to each other, and the final round is an intense slugfest between Mega Lucario and Mega Garchomp. Ash wins and advances to the finals with Cynthia pleased with his growth and wishing him the best. Also, they go get some ice cream together after the match.
- Ash vs Leon. Ash needs to take 2 matches to win, Leon only needs to win once.
- Ash takes the first match because Leon underestimates not Ash's skill or his roster's strength, but Ash's ability to think on the fly and get himself out of a jam, along with his creativity in using moves (which would likely get people crying more anime BS/plot armor/etc but screw it, I'm just dreaming here).
- Leon admits he underestimated Ash, and vows not to make the same mistake, going all out for the first time in what feels like forever to him. He proves to be an opponent with as much creativity and ability to turn bad situations as Ash, with their battle becoming an intense match that goes down to the wire. Win or lose, Ash would have not only proven his mettle, he would also have been the only one to have made the unbeatable champion sweat enough to go all out.
(*fanfiction mode off*) I sincerely doubt they'd put double elimination in the series ever, seeing how that's even more episodes needed to choreograph and animate and stuff, but I do wish it'd be there. Double elimination can be a really hype format.