My personal hope (I emphasize this so folks know I don't seriously THINK these would happen--it's just what I would do personally) involves Ash having a rotation team, and involving Pokemon of every gen, and every type:
Pikachu
Dragonite
Gengar
Riolu --> Lucario
Sobble --> Drizzile --> Inteleon
Grookey: I think Grookey will stay unevolved; the evolutions will be showcased by other characters
Tyranitar: A Gen 2 choice for Ash... it'd be even cooler if it were the baby Larvitar that Ash took care of from the Johto saga
Whismur --> Loudred: A Gen 3 choice for Ash. A cowardly crybaby that evolves in the face of danger to show off bravery
Swoobat
or Reuniclus: A Gen 5 choice for Ash. Caught in its final form like Dragonite/Gengar/Tyranitar
Swirlix: A Gen 6 choice for Ash. Remains unevolved; acts like a stereotypical dog, gets into a lot of mischief
Toxel --> Toxtricity-Amped: Starts off as a baby, evolves into an edgy Toxtricity--jams out with Loudred a lot
Snom --> Frosmoth: Snom has found some huge popularity, and I think it'd be cool to see Frosmoth with Ash
Kubfu --> Urshifu-Single Strike: I see Ash taking care of a Kubfu during an arc for the Isle of Armor; I don't foresee it sticking around--think Nebby
-Reserves-
Charizard: Returns for a few battles, maybe even Mega Evolves (Mega Zard-Y needs some redemption after its miserable treatment in XY league...)
Squirtle-->Wartortle: Returns for a few battles, evolves on-screen
Bulbasaur: Returns for a few battles
Garchomp: Evolved off-screen at Oak's Lab, returns for a few battles
The Alola Crew: No need to catch any new ones--he has his old Alolan pals waiting for him once they return to Alola (Naganadel can return for an episode or arc that involves the Ultra Beasts?)
I don't think we will see every, or even most of his Oaked reserves, but even just a few of the Oaked mons plus his Alolan pals would make me happy. I can't really see anyway that Ash would go back to Alola without interacting with Kukui or his old Pokemon. Plus we should see Giga-Melmetal and it'd make sense for Ash to utilize it.