I hate to be the bearer of bad news, but I think the list of returning characters in the Battle Tree will be *a lot* smaller than the PWT.
Remember that the PWT was back in the sprite era, so GF was able to just reuse Platinum and HGSS sprites for the Kanto, Johto, and Sinnoh characters. Only the Hoenn characters needed new sprites, which didn't take much effort at all.
Now that we're in the 3D era, bringing back older characters takes more time and effort. 3D models take longer to make than sprites, and they also take up more space in the game. Plus, only the Hoenn and Kalos characters even have already-made 3D models, so far less can be recycled from past games.
There's also the issue of Mega Evolutions. Wally is clearly going to have his Mega Gallade, and the other revealed trainers all have obvious potential Mega Evolutions (Cynthia = Garchomp, Red = Charizard X, Blue = Blastoise). Red even appears to be possibly wearing a Mega Bracelet in his new artwork. Megas may be last gen's thing, but if one character's going to have one, then it stands to reason that the others will as well, especially since the Battle Tree is a postgame facility.
Putting this all together, the two other characters I expect to return the most are Steven and Diantha. They're both Champions, they both already have 3D models, and they both already have Megas. Steven is also an extremely popular character like Red, Blue, and Cynthia. Diantha is less popular, but still has the advantage of being recent (and also conveniently adds another woman to the Battle Tree lineup).
Who I feel are much less likely to return are Lance, Wallace, Alder, and Iris. None of them have potential Megas (blame GF for shafting Dragonite, Milotic, and 99% of the Unova Dex when they doled out Megas last gen), and only Wallace even has a premade 3D model. They also don't exactly have popularity on their side, either. They've all long been seen as the "weakest" Champions, and it's possible that GF doesn't even see Lance, Wallace, and Alder as the "real" Champions of their respective games (instead favoring Red, Steven, and N, who all happen to be far more popular).
For those who don't believe GF plays favorites, remember Charizard, Mewtwo, and their two Megas each. They almost certainly pay attention to who is and isn't popular.
(And, I say this as someone who loves Lance very much and has gotten used to her favorite character always getting ignored and sidelined. :()