Well here's my prediction for Ash's Alola Team.
Ash
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Temporarily
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Pikachu:
Decidueye: I hope that Rowlet ultimately evolves all the way, I think it will evolve at some point on Akala Island and evolve into Decidueye on Poni Island.
Incineroar: I think that Litten has a good chance at evolving all the way and ill serve as Ash's ace. It will evolve into Torracat at some point on UlaUla Island and would then evolve into Incineroar against Nanu. Incineroar would go through some tension with Ash but after a while they would develop a close bond with each other.
Lycanroc: Rockruff is probably bound to evolve soon, I think he will end up evolving in order to help Ash beat Olivia's Lycanroc. After evolving its personality would remain the same.
Passimian: I think that Passimian will be Ash's next capture, Ash would meet a group of Passimian at Lush Jungle and one would specifically take interest in Ash. After beating it in battle with Pikachu, Ash would capture it and it would soon become one of the powerhouses on his team.
Sandslash: The last Pokemon Ash would obtain and his first steel type. It would be obtained as an egg from Sophocles cousin Molayne; and would later hatch later on Ula Ula Islan after the egg would be stolen by Team Skull. Sandshrew would be a shy Pokemon usually curling up in a ball but would be a capable battler being used to help Ash win his last Grand Trial. It would evolve into Sandslash on Mt. Lanaklla after finding an Ice Stone.
Solgaleo: I don't know how likely this is, but in the anime I am hoping that two Cosmogs are introduced one(Nebby) that bonds with Lillie and another one that chooses to follow Ash. Ash would meet a Cosmog that mysteriously came from The Lake of the Sunne/Moon and it would follow him around evolving into Cosmoem during the climax. It would then evolve into Solgaleo while Nebby would evolve into Lunala and together the two would help defend Alola from the Ultra Beast and Aether Foundation. Ash would command Solgaleo for a brief period of time before ultimately parting ways with it.
Pikachu: Thunderbolt, Quick Attack, Iron Tail, Electro Ball
Decidueye: Leaf Blade, Spirit Shackle, Brave Bird, Leafage
Incineroar: Darkest Lariat, Flamethrower, Brutal Swing, Flame Charge
Lycanroc: Accelerock, Crunch, Rock Climb, Stealth Rock
Passimian: Bulk Up, Double Edge, Close Combat, Energy Ball
Sandslash: Ice Ball, Metal Claw, Bide, Blizzard
Solgaleo: Flamethrower, Sunsteel Strike, Morning Sun, Hyper Beam