Not knowing how many Pokémon are to be used in each round is really messing up quite a few predictions as some of the competitor's teams are so limited.
The Top 16 consists of 1 vs 1 matches so that's okay. But if the Top 8 has 2 vs 2 matches then I can't see James or Lillie moving on since they only have 1 Pokémon on hand.
From what we know of the Top 8 is that Ash is facing Hau and that the match-ups are randomized because if they weren't Ash would have faced Guzma instead of Hau and Hau would face either Lana or Mallow.
So, assuming the Top 8 will be 2 vs 2 matches this is how I can see the league going.
Top 16 - 1 vs 1
Ash vs Faba (Meltan vs Hypno)
Ilima vs Guzma (Smeargle vs Golisopod)
Hau vs Samson (Raichu vs Vulpix)
Mallow vs Lana (Tsareena vs Primarina)
Sophocles vs Mina (Vikavolt vs Ribombee)
Jessie vs James (Mimikyu vs Mareani)
Kiawe vs Hiroki (Marowak vs Mudbray)
Lillie vs Gladion (Vulpix vs Umbreon)
Meltan could show off its 4th move, Thundershock and while fail to perform the Z-Move, it still wins.
Guzma will demolish Ilima to prove he's a real threat.
Hau vs Samson... I just want to see Vulpix battle even if it loses.
I want Mallow to pull of the Z-Move but Primarina barely tanks it to unleash her own move.
The same with Sophocles and Mina. I wanna see Twinkle Tackle but then have Vikavolt beat it with his move.
Jessie got the Contests and Showcases, let James get the Pokémon League, especially since he was so fired up to face Masked Royal. I can see Mareani beating Mimikyu with Knock Off and then evolving upon winning the match.
Kiawe vs Hiroki will probably be a quick match to prove he can beat Ground types.
Gladion vs Lillie will show some of Lillie's smart tactics but fail due to her inexperience.
Top 8 - 2 vs 2
Ash vs Hau (Torracat, Rowlet vs Raichu, Decidueye)
Lana vs Sophocles (Primarina, Eevee vs Vikavolt, Togedemaru)
James vs Guzma (Meowth, Toxapex vs Pinsir, Masquerain)
Kiawe vs Gladion (Turtonator, Charizard vs Zoroark, Silvally)
I don't want another Pikachu vs Raichu fight. Torracat should get a win here since Pikachu will be used for the battle royal. Maybe Torrcat wins with Inferno Overdrive and Ash can't rely on Bloom Doom to save Rowlet last minute so he has to go hard on the Feather Dance strat.
I was gonna have Lana vs Kiawe but I feel like Gladion taking about Kiawe would be the equivalent of the Paul vs Barry, Cameron vs Bianca, Tierno vs Sawyer matches.
I want Lana to train while Ash is training Rowlet. So during this match, Lana reveals she was training with Sandy to learn Dig. That's how she beats Sophocles and Togedemaru after Primarina and Vikavolt take each other out.
For Guzma's second match I want him to take out James. This will fuel TR even more to cheer for Ash in the finals. Since James only has his recently evolved Toxapex, Meowth decides to help him after being motivated by James and Toxapex's determination.
For Kiawe vs Gladion, I want Gladion to fully use Zoroark's Illusion so that Ash can think up of a strategy ahead of time. I can see Gladion using a Water Memory to counter Kiawe's fire types.
Then the Top 4 can be 3 vs 3.
Lana vs Guzma
(Lapras, Vaporeon, Primarina vs Masquerain, Scizor, Golisopod)
If Kiawe can use his Ride Pokémon, so can Lana. We know it at least knows Ice Beam. Plus she had Lapras before Popplio same as Kiawe and Charizard and Tortunator.
Water vs Bug is pretty even. I can also see Lana buying a Water Stone from a shop on the island. We already had Vaporeon hinted at us. Guzma's secret weapon would be a Toxicroak with Dry Skin that absorbs Vaporeon's Hydro Vortex and due to its typing, defeats Lapras AND Primarina as well.
Ash vs Gladion
(Torracat, Pikachu, Lycanroc vs Zoroark, Silvally, Lycanroc)
Ash's strategy for fishing out Zoroark's Illusion could probably be having Torracat tank one of two hits and have it use Revenge to deal super effective damage and weaken it so Illusion wears off.
Then Pikachu draws with Silvally to leave the Lycanroc duel the boys love to much but this time our Lycanroc wins.
The final match SHOULD be a 6 vs 6. It's only logical. I can see Ash calling Prof. Oak for a reserve or maybe Ash somehow gets Poipole back like he did with Goodra when the full battle came up.
Finals 6 vs 6
Ash vs Guzma
Rowlet, Torracat, Pikachu, Noctowl, Lycanroc, Meltan
Pinsir, Masquerain, Toxicroak, Scizor, Ariados, Golisopod
Rowlet vs Pinsir : Rowlet beats it with Brave Bird but is heavily damaged and gets recalled.
Torracat vs Scizor : Torracat gets hit with a U-turn and Masquerain comes out.
Torracat vs Masquerain : Masquerain uses Rain Dance and Bubblebeam to heavily damage Torracat. Torracat gets recalled and so does Masquerain.
Pikachu v. Ariados : Electro Web vs Spider Web collision. Pikachu beats Ariados and gets recalled.
Torracat vs Scizor : Torracat still heavily damaged by Masquerain's surprise Water-type moves draws with Scizor after a Bullet Punch Flame Charge collision.
Brief Break.
Ash has 5 Pokémon
Guzma has 3 Pokémon
Rowlet vs Toxicroak : Toxicroak makes quick work of Rowlet.
Noctowl vs Toxicroak : Ash's secret weapon was his Noctowl which can use Psychic-type move as they are extemely effective against Toxicroak. Noctowl wins but gets Poisoned during the fight.
Noctowl vs Masquerain : Masquerain KO's Noctowl with a surprise Ice Beam.
Lycanroc vs Masquerain : Lycanroc wins with an Accelrock but not before another Rain Dance is pulled off.
Lycanroc vs Golisopod : Golisopod takes an Accelrock but OHKO's Lycanroc with Liquidation under the rain.
Pikachu vs Golisopod : Ash attempts to finish it with a Gigavolt Havoc but gets hit with a surprise Drill Run after shooting the move off. Guzma got inspired to teach Golisopod Drill Run after seeing the Steel-type Corkscrew Crash used by Pikachu earlier.
Meltan vs Golisopod : Down to his last Pokémon after being outplayed by Guzma in the second half, Meltan maximises its defense with Harden and barely tanks a Drill Run. It then summons the Meltan army and they all fuse together to evolve into Melmetal. Melmetal faces off with Golisopod with a newly learned Thunder Punch. A few collisions happen and then Melmetal overpowers Golisopod and beats it.
Ash becomes the Alola League champion.