Can't see Steven & Diantha not being in the Top 8, so I'd substitute out Paul & Alain for Steven & Diantha.
Not sure if I can see Iris being there either, but I wouldn't rule it out. Would give us representation from all regions (with Ash representing Alola as the champ and Lance representing Kanto/Johto, with the rest being self-explanatory). Could also see another Galar Gym Leader being there (not Bea, hopefully). Ultimately, I'd like to see the finals be something like this:
Quarter-Finals
Ash VS Raihan
Cynthia VS Lance
Steven VS Diantha
Leon VS Iris
Could substitute Iris for someone else and depending on who, I could see that person fighting Steven instead and then Diantha fighting Leon. Ultimately, I think Leon should face either 2 champions before Ash or a Galar Gym Leader and 1 champion before Ash.
Semi-Finals
Ash VS Cynthia
Leon VS Steven
Finals
Ash VS Leon
Also, I hope Steven & Diantha Mega Evolve their Metagross and Gardevoir respectively. Would be cool if Cynthia and Lance do that as well with her Garchomp and maybe his Aerodactyl (he hasn't been shown to have one in the anime, but I could see that being on his team in a 6 on 6).
Since the anime is taking place during gen 8, I feel like Ash has to fight Raihan, and I feel like fighting Leon is a given. This is why I hope Bea is not in the Masters Eight because then he'll likely fight her as well, meaning he'd never fight Cynthia. I want him to fight Cynthia using his best 6.