I liked Zenos' character design and English voice, but I never felt that Zenos was a "rival". There is no relationship or competition between him and the WoL. It is not like they were both students under master Musosai, or both junior adventurers at the same guild branch in Limsa/Gridania/Ul'dah before parting ways. He is just some enemy general you were working against and only bumped into a couple times before the allied forces invaded his territory and you dealt the coup de grace. Then later he gets rezzed and talks about fighting you... but spends years sitting around doing nothing. That is not a rivalry. That is a wasted character and plotline.
However, I did like the idea that there were other people out there in the world doing their own thing and with the potential to shape the world independent of what we are doing and the
characters we are interacting with . The Zenos scenes starting in the post Stormblood patches were interesting because of that.