It's possible that some of the original Naruto characters are still main characters but are now involved in storylines related to the 'lightning bloodli e'. For instance, Naruto himself might encounter a character with this special ability and have to either fight or team up with them.
One interesting concept could be unique jutsu related to lightning and blood. Maybe there are new ways of combining these elements in combat that are different from what we see in the original Naruto series.
Well, Naruto is most likely one of the main characters since it's in the title. There may also be other characters from the Naruto series like Sasuke or Sakura. And if it's about a lightning dragon slayer, there could be new characters introduced, perhaps a mentor - like figure who teaches Naruto about slaying lightning dragons.
Naruto himself is often a popular character in such fanfics. Since he has a natural affinity for different types of chakra, it's interesting to see how he would adapt to the lightning sage powers. Another could be Sasuke. His sharingan and his quest for power could be combined with the lightning sage abilities in an exciting way.
Well, clearly Naruto and Anko would be the main characters. Naruto is the protagonist of the original Naruto series, known for his determination and his powerful jutsu. Anko is a strong kunoichi with her own set of skills and a bit of a wild side. In a fanfic focused on them and a spirit theme, they would be at the center of the story.
Obviously, Naruto and Hanabi are the main characters. Naruto, the hero of the original Naruto series, and Hanabi, an important member of the Hyuga clan.
Often, the main character could be an original character created by the fanfiction writer who has a special connection to the crimson lightning. Maybe they are a young ninja trying to prove themselves in the Naruto world.
The main characters would be Naruto and Erza. Naruto is known for his rasengan and his ability to make friends. Erza is famous for her various armors and her leadership in Fairy Tail. In this fanfic, their spirits interact, so they are the central focus. There might also be some side characters from their respective universes like Sasuke from Naruto's world or Gray from Erza's world who could play supporting roles.
Since I don't know the specific 'lightning dragon tail fanfic', it's hard to say. But it could be the dragon itself as one of the main characters. There might also be a dragon rider or a wizard who has a connection to the dragon.
It might be an existing character from the Naruto series who has a new encounter that leads them to focus on the black lightning. For instance, Sasuke could stumble upon a hidden scroll about the black lightning and decide to master it to become even more powerful.
Obviously Naruto is one of the main characters since it's in the title. And Lightning Farron is likely to be a main character too, as Naruto is supposed to be his son in this fanfiction. There may also be other Naruto - universe characters like Kakashi or Jiraiya who could play important roles in the story.