It could be Orochimaru. Imagine if he also found a way to cross over into the Avatar world. With his knowledge of dark jutsu and his desire for power, he would be a formidable antagonist. He might try to use the bending abilities for his own evil purposes and also target Naruto because of his immortality, seeing it as something he could steal or use to further his plans.
Maybe a combined form of Madara Uchiha and Ozai. Madara's power - hungry nature and Ozai's thirst for domination could merge into one terrifying antagonist. This combined villain would want to rule both the Naruto world and the Avatar world. They would see Naruto's immortality as a challenge and the Avatar as a threat to their plans of total control. They would start wars in both worlds and force Naruto and the Avatar to team up to stop them.
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.
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.
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.
One possible plot could be that Naruto, with his newfound immortality, somehow gets transported to the Avatar world. He could meet Aang and learn about bending. Naruto's chakra control might help him quickly adapt to bending techniques. He could then team up with Aang to fight against a common enemy, like an ancient dark spirit that threatens both worlds.
One possible fanfiction could be about Naruto characters finding themselves in the Avatar world. For example, Naruto's determination and never - give - up attitude could be a great addition to the Avatar universe. He might learn bending techniques in a unique way, perhaps by combining his chakra control with the flow of chi in the Avatar world. This could lead to some really interesting battles and character interactions.
The crossover could start with a strange portal opening between the two worlds. Naruto characters like Sasuke might be intrigued by the bending abilities in the Avatar world. He could try to learn firebending, thinking it could enhance his own fire - based jutsu. And the Avatar characters could be equally fascinated by ninjutsu.
Madara Uchiha could still be a great antagonist. Even with Itachi and Naruto teaming up, Madara's power and his long - term plans for the ninja world would pose a huge threat. His knowledge of the Sharingan and his connection to the Uchiha clan would make for an interesting conflict against Itachi, while his overall strength would be a challenge for Naruto as well.
A new character could be created. This character could be a powerful slug - human hybrid who has been corrupted by the dark side of both worlds' powers. He wants to rule over both Naruto's world and Slugterra, believing that the two worlds should be united under his rule. He has the ability to use both ninja techniques and slug powers, making him a very difficult opponent to defeat.
Perhaps an ancient and evil ninja from a long - lost clan that was sealed away behind the gate. He resents the modern ninja world and wants to destroy it. He has unique and powerful jutsus that Naruto and his friends have never seen before.
It could be someone like Doctor Octopus who gets his hands on some ninja technology. He could use it to enhance his mechanical arms and also start stealing chakra from people to power his new devices. His scientific mind combined with the newfound ninja powers would make him a formidable foe for Naruto and Spiderman.