Yeah, Serena deserves a comeback as well. But they had to give a reason for the trip, maybe reintroduce the Pokemon contests/showcases to the Alolan classmates in which May & Dawn might be able fit in as well.
As for Iris, a dragon-type gym or museum visit might be good reasons for her cameo, or have one of her Pokemons participate in a competition where the Alolan classmates can participate as well.
For Cilan, well he got involved into a fishing competition and can cook, which can benefit Lana & Mallow respectively.
For Clemont, he's doing those robotic inventions in which can benefit Sophocles.
As for Max & Bonnie, they'll probably going to stick with their siblings.
That's some ideas I got in my mind, but I don't think any of them would make cameo appearences in SM though.