Kanto
After Brock and Misty, I don't think he'd win another badge as a handout. Also, the gym battle episodes would have been much better if done today. I probably would have had Ash actually face Giovanni also rather than the TRio.

Johto
There's a lot I'd change. First of all, I'd cut a handful of filler episodes and fill them with episodes dedicated to Brock and Misty, Ash training his Pokemon, Gym Leader intros, and more rival related episodes. Speaking of rivals, I'd give more focus to Gary and Casey, giving Ash and Gary's rivalry battle more build up. Plus, I'd have him battle Casey a bit more and have her as a league rival. I'd probably introduce Jackson and Macy sooner also. I might take that one trainer with the Skarmory used to develop Cyndaquil a bit and build that into a rivalry so Ash could actually meet that trainer and face her in the first round of the Silver Conference. Lastly, I'd change up Ash's team a bit more. I would have had the Kanto starters take a back seat sooner. His team would end up as Pikachu, Noctowl, Meganium, Quilava, Heracross, and Espeon. Totodile would have gone to Misty and help develop her more as a Water type master.

Hoenn
There's not much I'd change. Hoenn is great, though I would have Wally be a rival for Ash in a similar manner to Sawyer where Ash is the one defeating him, only to lose to him before the league, then beat him at the league.

Sinnoh
This would be Ash's swan song. Here, it would be the culmination of everything he's learned, allowing Unova to be a true reboot. Not much would change, though some things such as Torterra's usefulness would change, and there wouldn't be as much of a gap between Ash's battles with Candice and Volkner. As for how things could change, I might have Dawn win the Grand Festival, allowing her to defeat Zoey. Plus, I'd actually give an episode or two more dedicated to the league, that way we can show off more of his older Pokemon and see more battles, like getting to see Quilava's battle or Snorlax's battle. Ash would still go on to battle and defeat Paul though things with Tobias would change. Tobias would have more build up and backstory before the battle, where Ash would actually beat him and give him the chance to take on the Champion's League and MAYBE Champion Cynthia.

Unova
Here would be the true reboot. We'd be following an entirely new trainer, though I'd still give them a Pikachu. I'd still have Ash show up on occasion to meet the new protagonist and maybe help them out. I'd still go with the Team Rocket vs Team Plasma plot and maybe keep the TRio around a bit longer. I probably would still stick with Cilan and Iris also. I'd make this protagonist lose to Virgil rather than Caneron also, then they go onto Kalos.

Kalos
I'd stick with the same protagonist from Unova, and probably have them also meet up with Serena, Clemont, and Bonnie. I'd give Clemont more to do in the second half of the series, and probably still keep Alain. I'd have the new protagonist beat Alain also in the league. I'd probably do more with Team Flare before the big finale also.

Alola
I'd probably give the SM series a new protagonist, going with the USUM male protagonist this time. I'd change things up just a little bit. I'd stick to the school element, though I'd give the Island Challenge significantly more importance. When it came time for a trial, I'd have the class stick around the island a bit more and do a bit more traveling to reach the next trial. I'd probably have more Team Skulls stuff going on and introduce Guzma a lot sooner. The Alola League would largely be the same also.

Galar
No Journeys this time. No Pokemon Go influence. No Goh and his uninteresting goal. Just Galar. All Galar.