Ash's rivals/opponents only. (At least, the ones I remember off the top of my head.)
Ritchie: Drop the Ash's Pokémon were tired drama. Disqualified for disobedience is fine, disqualified for sleep is not.
Gary: More appearances, backstory, and relationship with Ash. I understand why we didn't get it in Kanto, but he should've shown up more in Johto. Admittedly, I don't hold this too hard against the writers, mainly because it was their first time and they got better later on.
Harrison: Did his job of advertising the third gen. Should've shown up in Hoenn, if not as a league competitor, at least as a cameo.
Katie: Promoted to main Hoenn rival. How is she still the coolest girl Ash has ever battled?
Morrison: His conflict felt unearned. If he had been a friend of Ash's before the league I could buy it.
Tyson: He's fine. I like how Ash isn't the only trainer with a mascot Pokémon. Once again, more appearances before the league wouldn't have hurt him.
Paul: Almost perfect rival. Maybe tone down the Chimchar abuse and the Maylene drama, basically make him less "evil".
Conway: Did his job. Mostly fine.
Nando: See above. Also fine.
Barry: Takes out one or two of Paul's team in his final battle. Magmortar had no business tanking Hydro Cannon.
Tobias: Either give him backstory ("this is why/how I have legendaries" or "here's a regular Pokemon I used to catch the legendaries") or acknowledgement ("Darkrai trainer lost to E4/Champion" or "he opted out of fighting the E4/Champion and vanished"). Note that Ash was the only league trainer to take down two of his legendaries, maybe a throwaway line about Paul having the potential to down at least one as well.
Trip: Either make him experienced (or at least not a new trainer) or make him an Ash-counter like Sawyer. Give him his 6v6 at the league.
Bianca: She's not fine, but needs Cheren to play off of to be significantly better. Maybe a 3v3 with Ash outside of the league.
Stephan: See Conway/Nando. Give Zebstrika some electric moves.
Cameron: Delete him. Tone done his stupidity. Make him like that guy from Haikyuu, a genius in battling, but kind of dumb outside that niche.
Virgil: Give him a Sylveon to promote fairies and gen 6.
Sawyer: Appears earlier in XY. Make him at least have traveled through Hoenn.
Alain: Make him fight Gurkinn/Korrina/Wulfric earlier, so he has a few badges before Ash convinces him to join the league. Gives Ash the keystone/Charizardite X he got from Lysandre after the TF arc. (Imagine the salt lol.) He's fine as the Kalos dream crusher.
Gladion: Has 6 Pokemon by the league. Full battle against Ash.
Kiawe: More interactions with Gladion before the league. (I really liked their matchup.) Has a real 3v3 with Ash post-league.
Hau: More appearances pre-league. At least a 3v3 battle with Ash. Give him Decidueye's or Alolan Raichu's Z-move.
Faba: 3v3 with Ash. Psychic Z-move. Let Meltan evolve earlier and solo his team.
Guzma: More appearances pre-league. Mostly fine otherwise.
Sophocles, Lana: More captures and onscreen battles. Let them do a grand trial.
Mallow, Lillie: More onscreen battles. Let them do one or two regular trials.
Kukui: Let us see more of his team pre-league, and maybe let him catch Tapu Koko and get Tapunium-Z before the league as well.