Shouldn't Wally be older? Since Hoenn and Kanto's games takes place within the same timeframe, he can't be that much younger than Red and Blue. If Red and Blue are 20, then Wally should be around his late teens during SM.
Seeing as Cynthia's in the Battle Tree and it mentions famous trainers, that should of course mean Lance, Steven, Iris and Diantha should be available. Wallace and Alder are a possibility, but I doubt the latter will appear due to abdicating his position after BW. I'd like to see the protagonists too, but with mixed genders for diversity (Ethan, May, Dawn, Hilbert, Rosa, and Calem for example). If Wally and Blue are in, then perhaps the other rivals, so Silver, May/Brendan, Barry, Cheren/Bianca, Hugh, and maybe Shauna since she mentioned going to another region.
This goes without saying, but if most of the pre-gen V Gym Leaders were in the PWT, they should be in the Battle Tree. I'd love to see Roxie, Marlon, and the Kalos leaders added into the leader roster.
This whole thing...