My predictions for the final matches (considering that it's a Tournament format, though I still have a little hope for Round-Robin format).
Quarterfinals:
Ash vs. Iris - Ash wins
Alain vs. Steven - Alain wins
Leon vs. Lance - Leon wins
Cynthia vs. Diantha - Cynthia wins
Semifinals:
Ash vs. Alain - Ash wins
Leon vs. Cynthia - Leon wins
Final:
Ash vs. Leon - may be either, depending on the writers' plans for the future. My heart says "Ash" (partly because I want the new anime series to finally end Ash's story. Also, I believe that Goh might have been introduced to see how popular a non-Ash protagonist could be) but my brain says "Leon".
I know that these matchups are boring as hell, this is why I predict them to be real. Journeys writers seem to hate Lance, I can totally see him getting smeared by Leon's Charmanderino once again.
I think that Ash's opponents are pretty much set in stone. Iris, Alain and Leon, who will definitely be the final match.
Iris is Iris - they didn't bring her here to make her stand in the background. Alain is here essentially to get beaten by Ash - rematch for the long-remembered Kalos League Final, so this one is obvious. Leon is even more obvious - the entire Journeys lead up to his final match with Ash.
As for the other Trainers, they're here mostly for flavor. They're just NPC Champions from previous regions and don't have much business to do with Ash. They are here because they are implied to be the world's strongest Trainers in the games, so they are obvious choices for such an ultimate challenge. But I doubt any of them will battle Ash, unless it's a round-robin.
Of course, my predictions for non-Ash matchups are debatable. It can be Alain vs. Diantha, Cynthia vs. Lance (if they don't want to make yet another Leon/Lance battle), Leon vs. Steven, for example. But as for Ash's opponents, they seem to be decided, I don't see other possibilities.
Also, I don't understand why everyone and their aunt predict Ash to battle Cynthia. Because she's popular? Yes she is, that's why she's here in the first place. But she's not the main dish here. Because of BDSP? The writers have already done their quota of new game advertising with those (outrageously awful) Arceus specials and the Dialga/Palkia two-parter. They don't seem to care about this too much, either.
Those were predictions, dictated by common sense (they're not preferences or hopes - actually, I'd prefer Ash NOT to battle Iris). But if I could state my wishes, I'd want Ash to battle Lance. Firstly because it's never happened in the past, secondly because I'd like to see what Pokemon they'd give him aside from Gyarados and Dragonite, especially if it's a Full Battle (though I'm pretty sure it'll be all 3-on-3 battles, maybe except the Final Battle, but I'm not sure about this either). Of course this is not happening in Tournament format, because of the above mentioned reasons, but maybe we'll at least catch a glimpse of Lance's other Pokemon in some quick montage of non-Ash Tournament Battles (or at least on the results screen). Generally, I'm hoping to see some previously-unseen Pokemon for all those Trainers, not just their aces. Another thing I'd love to see is Porygon-Z in Cynthia's posession. Don't count on a big appearance, it can be just a short scene of it beating (or getting beaten by) its opposing Pokemon. But I'd love Journeys to acknowledge Porygon-Z's existence, especially since Journeys has done so many things that previous series neglected to do (e.g. giving Ash Mega Evolution, showing some previously-unseen locations and characters, showing a proper PokeMart with the clerk etc.). But those are wishes, not predictions. I doubt either will happen.