I think it'd be better to guess who's in the other classes rather than who would be in the Masters Eight. With there being Leon, Lance, and Raihan, the rest will probably be either givens or complete surprises like Flint and Tobias. The givens would most likely be Cynthia, Steven, and Diantha at the least. Alder doesn't seem like the type to be part of it, given that's he's... incompetent, for lack of a better term given his behavior in both the games and the show.
Some Ultra Class trainer I'd love to see are Paul, Virgil, Brandon, and a surprise guest debuting would be Benga from BW2. Maybe even Misty!
Other trainers I could see in the Great Class are Kiawe, Lana, Nando and Sawyer. Perhaps Wally from Hoenn could make his surprise debut here.