Jangobadass
Fear the Chicken!
Gary cameo would be nice to see how he developed as a Pokémon Professor in training, referencing to his in-game appearance at Battle Tree. Not expecting Battle Tree to be a part of the anime at all, just visiting Sam Oak in Alola to investigate the Alolan forms and have a friendly match against Ash.
And maybe debuting a Galar Pokemon to send Ash to Galar the way he sent him to Sinnoh