Soniman
Break the Limit
This would be a nice surprise. Even though that said character is not on the voice cast list, it could still appear as a brief cameo (like Kenzo (Keenan), Goodra (before rejoining Ash's Team), and other Wetland Pokemon watching the Pokemon Showcase).
I still find it funny that even in XY verse characters like Korrina, Viola and Alexa bothered getting cameos for Serena's master class but not Ash's League, even though all those characters have a way more significant relationship with Ash. They could at least give us Sanpei or something