I think the ship of getting a new lead character for the Pokemon anime has long since sailed, though I'd be down more side stories like the Pokemon Chronicles series to get the supporting characters a little more screen time. If they were to go with a new main character though, I'd hope for a brand new character. Maybe Hau, since I don't believe he's been introduced in the Sun/Moon anime yet- they could follow his story of completing the trials of Alola, possibly meeting with Ash along the way.
Anyway, this is more suited over in our Pokemon Anime board, so I'll move this over there for you.
MOVED.