Nyx was an add-on character, for the movie, with very little relevance to the overall plot. Cor had unexplored relevance throughout Final Fantasy XV. And just look at that face. That face which just screams of regret, hardship, and undue sadness. Cor could have had a friendly rivalry with Titus Drautos who turns out to be General Glauca.
Cor is the “unlucky” character (with his age as being 42, an unlucky number in Japan). Making it to Lunafreya while wearing the ring (to give to her) would make him very unlucky indeed.
I think Cor’s story could have worked for Kingsglaive: Final Fantasy XV. He’d be a legendary Kingslglaive promoted to Crownsguard. He wouldn’t die at sunset but at an unexpected time. Why else would Cor be so unlucky?
Throughout his life, Cor survived countless battles, earning him the moniker “Cor, The Immortal”. He dedicates his life to serving Lucis.
What if he were the “Immortal” due to some other unlucky reason. Wouldn’t have to have anything to do with the ring, but something with the Lucii Kings beforehand. Being unlucky enough to see three dynasties (Mors, Regis, and Noctis) simply isn’t enough.
He knows he isn’t going to be with Noctis for long due to the orders from Regis as to the Crownsguard’s new purpose, and warns him not to get too comfortable with the idea of having him around.
Because he knows the same will happen with King Regis?
Nothing against Nyx. He just a prop for the sake of a just movie. What if Cor had been there as the main protagonist to tie in the movie and the game?
Now, of course, they’d have to make many scenes of Kingsglaive: Final Fantasy XV and Final Fantasy XV. And this doesn’t necessarily mean that Cor’s corpse would have dangled in front of Noctis…although it would provide more power to that scene rather than if Nyx were in it:
Noctis: Oh noes! There’s my dad, my wife to be, the evil king…and that one guy who I think drove me to a theme park that one time…
Clearly, Noctis didn’t see the movie. He was too busying away, fishing.
Either way, I just would have liked to see more of Cor. Maybe Nyx could have been his understudy, or his Robin, per se…
Maybe when, or if, they get around to doing his DLC we’ll have more information on him.