Viola would have made sense. Ash could have beaten her and Viola's goal could have been to become a Grand Duchess in the Chateau. She has an extremely outgoing personality, we know one of her relatives, she could have been the older member of the group, and unlike Bonnie, she could have had a goal. Korrina could work, too, but I worry that if her arc is any indication, Lucario would take away a lot of screen time from Clemont and Bonnie.
Yeah, me, too. Rivals work well when they are competitive with each other.