Cosmic77
Smash Hero
Honestly, I think I'd prefer more of a plot, better battles, and a more purposeful role for Team Rocket over old characters returning.The thing about a few past characters is that we need to fill up who the remaining top 4 of the World Coronation series will be. We know for certain Leon, Raihan and Lance will be part of it. And of course Ash would have to be there as well leaving 4 spots. Some candidates i can think off the top of my head are: Cynthia, Steven, Alain, Diantha and maybe Alder.
As for the others there really isn't much need to have a lot go on in their return episodes, it can just be a situation of "Hey, long time no see. How have your been?" and it's more for catching up on things or just a silly little plot. Not saying it's guranteed that past companion will return, but the fact that a lot of fanservice is showing up in this series, namely Ash getting Dragonite, Gengar & Riolu, tells me it's possible now more than ever to bring them back.
Maybe not now, but perhaps next year or next season.
Don't get me wrong, it's nice seeing Ash with better Pokemon and old characters making a return, but I feel like there are so many existing characters and Pokemon in the current cast who they desperately need to flesh out more. Chloe is a prime example, as well as any Pokemon who isn't Pikachu, Riolu, or Raboot. They're also introducing the Galar content at a snail's pace, so we have PLENTY of characters, locations, and Pokemon who should be prioritized over those who have already been featured in the anime in previous series.