Serena could meet Ash after arriving in Luminoise. She reveals she's been a trainer for quite some time and mainly had a lot of experience before she become a trainer using the family Rhyhorn. Revealed to own a Gogoat who she controls very well due to the experience with Rhyhorn because they have similar frontal battling styles.
Honestly, it's hard to see any of the XY crew as serious rivals without a personality transplant. Tierno, Shauna and Trevor aren't striving for the same thing as the player in-game nor do any of them seem competitive like Georgia despite not going by the rival rulebook definition. I don’t like to use the term rival for them anyway since they’re a bunch of cute dorks being friends and starting their journeys together and occasionally testing each other’s skills in battle for fun. I don't see any of them being mains and if it's not Serena or Calem then I think we'll get an OC rival.