It could be a rogue ninja who also has a devil fruit power and wants to steal Naruto's Mera Mera no Mi ability. Maybe he has a water - based devil fruit power and sees Naruto's fire - based power as a threat to his own plans for domination.
Perhaps in the fanfiction, the Mera Mera no Mi's power is modified when Naruto has it. Since Naruto has a large amount of chakra, the fire produced by the fruit could be enhanced in a way that it can even burn through powerful ninjutsu defenses. For example, when facing Sasuke's Amaterasu, Naruto's Mera Mera no Mi - enhanced fire could counter it in a unique way.
I really like the fanfic 'Aquaman and Mera: United Front'. It's a story that focuses on their teamwork. They go on various adventures and battle against different threats to Atlantis. The author does a great job of portraying their individual strengths and how they complement each other. There's also a fanfic called 'Mera's Vision'. It shows Mera having a premonition about a great danger to Atlantis and she convinces Aquaman to take action. It's full of suspense and their relationship is a central part of the story.