Guessing Piers or Kabu. Would be fitting and would give some additional Galar representation other than Leon and Raihan considering we're in Gen 8 right now.I feel like Cynthia and Steven are shoe-ins.
I bet you at least one other Galar gym leader will get thrown in if it isn't just champions. Probably Piers tbh.
And maybe one of the conference winners, (not John Dickson or that random guy that won the Indigo League).