Sasuke could also be a main character in some Naruto Phenex fanfictions. He might be interested in the Phenex's power and see it as a way to become stronger. His interactions with Naruto in the context of the Phenex clan's powers and secrets could add a lot of drama and action to the story.
Then there could be a Phenex clan member as a main character, perhaps a young Phenex who is on a journey and crosses paths with Naruto. This Phenex character could have unique abilities like fire manipulation and regeneration, which are typical of the Phenex in some fanon interpretations. They could befriend Naruto or start off as rivals.