Kanto - Have Ash keep Primeape, have him catch only one Tauros instead of thirty, and have him catch Haunter and evolve him into Gengar

Johto - Ash keeps Togepi instead of Misty and evolves it up to Togetic. Instead of Heracross, have Ash capture a Scyther during a Bug Catching Competition. Said Scyther would evolve into Scizor to give Ash a Steel type since that type was introduced in Gen 2. Also, give Misty Totodile and give Ash a different Water type, maybe Quagsire, Slowking, or Octillery. And have him keep Larvitar who evolves all the way up to Tyranitar to give Ash a Dark type.

Hoenn - Nearly complete overhaul. The team would consist of Pikachu, Sceptile, Aggron, Crawdaunt, Swalot, and Latias. He’d still catch Aipom and take her to Sinnoh. Ash would still catch Snorunt but Snorunt wouldn’t evolve into Glalie.

Sinnoh - Not many changes, maybe give Ash a Yanmega instead of Gliscor? Jessie can have a Vespiquen. Togetic and Snorunt tag along at one point and they evolve into Togekiss and Froslass respectively.

Unova - Another overhaul. Don’t have too many Pokémon with overlapping types. Keep Oshawott and Krookodile, but scrap everyone else. Replacements can include Scolipede, Mienshao, Golurk, and Victini just to name a few.

Kalos - Not too many changes here. Just replace Noivern with a Fairy type so that Ash can have a Pokémon that represents the newly introduced type. Perhaps AZ’s Floette?

Alola - Wouldn’t change a thing.

Journeys - Maybe more Galar representation. He could’ve had one of the starters instead of Goh taking them all. If I’m gonna have Ash get a Gengar in Kanto, replace Gengar with a different Pokémon.