Naruto himself could be a great protagonist. Imagine if he somehow gained the powers of the Lightning Avatar in addition to his own Nine - Tails chakra. He would be an even more formidable force, using his never - give - up attitude to master the new powers and protect the Hidden Leaf Village from threats.
One idea could be a story where the main character has the power of the Lightning Avatar and is sent to the Naruto world to help in a great war. They could use their unique lightning - based abilities to turn the tides of battle against powerful enemies like the Akatsuki.
Another possibility is a powerful spirit from the Avatar world that has been corrupted by the introduction of Naruto - like powers. This spirit could resent the intrusion of this new power source and seek to destroy anyone associated with it. It might have unique abilities that combine the spiritual magic of the Avatar world with the raw power of Naruto's chakra, making it a formidable foe.
It could be a rogue ninja who has somehow harnessed the power of the Crimson Avatar in a dark and corrupted way. This ninja could use his powers to try and take over the ninja villages, believing that the traditional ninja ways are weak. He might see himself as a new kind of leader, using fear and his overwhelming Crimson Avatar powers to rule.
In the 'immortal naruto avatar fanfiction', the main antagonist might be a corrupted Avatar. This Avatar, instead of maintaining balance, wants to use Naruto's immortality to become an all - powerful dictator. He or she would have the bending abilities of an Avatar and the knowledge of how to use them in devious ways. This would put Naruto in a difficult position as he has to either stop the Avatar or find a way to redeem him/her while also protecting his own immortality.
Maybe an ancient being that was sealed away long ago but has now been awakened due to Naruto's newfound power. This being has a natural hatred for those who can control lightning and sees Naruto as a threat to its plans of taking over the ninja world. It has the ability to manipulate shadows and uses them to counter Naruto's lightning attacks.
A rogue ninja from a neighboring village who has also mastered a similar but dark - powered version of the Lightning Rasengan. He could be jealous of Naruto's fame and power and wants to prove that his version is superior.
Maybe it's Naruto himself with an additional power - up. Imagine Naruto gaining the godlike lightning ability on top of his existing powers. He would be even more formidable and could use the lightning in combination with his shadow clones and Rasengan techniques. This would change the dynamics of his battles and relationships with other characters like Sasuke, who also has powerful lightning - based attacks.
It could be a rogue ninja who was once rejected from the lightning sage training and now wants to destroy the entire concept of the lightning sage. He might have developed his own dark lightning - based techniques to oppose the traditional ones.
It could be a rogue ninja from a hidden village who has also mastered lightning jutsu and wants to prove himself stronger than Naruto. This ninja might have a personal vendetta against Naruto or Konoha.
It could be Orochimaru. He might have learned about the Avatar's unique power and wants to use it for his own dark experiments. So he tries to break Naruto's seal to get to the Avatar. He would be a great villain as he's always after powerful things and doesn't care about the consequences.