So we've seen the return of Iris & Korinna for a PWC battle and Drayden, Chuck, Kakui, Gary, Mallow, Lana, Sophocles, Kiawe & Burnet as non participants. So seems the non pwc group is significantly biggerIt was only the Alola cast and Gary. Everyone else who returned did so as a PWC participant.
If they want to bring back May, Dawn or Serena all they have to do is an episode about contests in which Go or Chloe wants to see one and one of them happens to be participating in it. As for Bonnie, Clemont and Max, all they have to do is have an episode in the town their gym is located in (or Normans gym in Max case)
So yeah, most old companions can easily have an appearance without the pwc. Especially since Ash has a "home base" now, so they can even drop by for a visit