He could be a hero, of course. Naruto has a strong sense of justice, so in a Marvel fanfic, he'd likely be fighting against the bad guys. Maybe he'd team up with the Avengers to take on Thanos - like threats.
His role might be that of a bridge between different power systems. Since he has Sage powers from his own world, he could introduce the concept of chakra and natural energy to the Marvel characters. He could teach them new ways of fighting or sensing danger. For example, he could show Spider - Man how to better sense his surroundings using chakra - like techniques. Also, he could be a source of inspiration for characters like Daredevil, who rely on their senses. He could show them new ways to enhance their already acute senses.
Well, Naruto's Sage Mode gives him enhanced physical abilities and the ability to sense chakra. In a Marvel fanfic, if combined with Marvel powers, it could be really interesting. For example, if he had some sort of connection with the Hulk's gamma - based powers, his Sage - enhanced strength could reach new levels. Maybe his chakra sensing could be used to detect other types of energy in the Marvel universe, like the Cosmic Energy.
Sage Naruto could be a mentor - like figure. He has a lot of combat experience and knowledge of chakra that he could pass on to the Titans.
In this fanfiction, Naruto Sage could become a mentor figure. He has a lot of knowledge and skills that could be passed on to the characters in Westeros. He could teach them about discipline, as he was trained rigorously in the ninja arts. Also, his sage mode gives him insights that could be useful in understanding the magic and mysteries of Westeros. For instance, he might help Bran understand his new powers better. He would be like a strange, yet powerful, new addition to the already diverse cast of characters in GoT.
Ahsoka Tano might be a lone wanderer in the Marvel universe in a fanfic. She could be on a quest to find a way back to her own universe, but along the way, she gets involved in various Marvel storylines. For example, she could stumble upon the X - Men and help them deal with a mutant - hating group. Her combat skills and ability to sense danger would be useful in protecting the mutants.
If Naruto was in the Marvel universe as an immortal, he might be friends with Spider - Man. They both have a sense of humor and a strong sense of justice. Naruto could teach Spider - Man some new combat moves using his ninja skills. He might also respect Captain America for his leadership and unwavering beliefs. Naruto's immortality could make him a sort of mentor figure to the younger heroes in Marvel.
His positive personality would make him an excellent servant. He's very determined, so he'd do whatever it takes to fulfill his duties. His friendly nature would also mean he could get along well with his master and other servants.
In some fanfictions, his father, Minato, might be a guiding spirit or a source of hidden knowledge for Naruto as the Six Path Sage. His mother, Kushina, could be a protective force, perhaps using her unique chakra abilities to assist Naruto in his new, powerful state. For example, Minato's Flying Raijin technique knowledge could be passed on in a more spiritual way, and Kushina's strong will and Uzumaki clan powers could be an added boost to Naruto.
Naruto might serve as a trainer for the younger or less experienced ninjas during the war. He could pass on his skills and knowledge, not just in terms of jutsu but also in terms of the mental strength needed to survive in a war. This would show his growth as a character, from being the one who was always learning to now being the one who imparts wisdom.
They could use the Rinnegan's power to gain an edge in battles. For example, the Asura Path could be used to create powerful mechanical weapons to fight against Marvel villains like Thanos. It would be a great way to show off the unique abilities of the Rinnegan in a new and different universe.
Batman in a Marvel fanfic would be a very interesting addition. His detective skills are top - notch, which could rival characters like Daredevil. He might have a hard time adjusting to the more super - powered and otherworldly elements in the Marvel universe compared to Gotham. But his intelligence and resourcefulness would make him a valuable asset to any team, perhaps even working with the Avengers on a mission that requires his particular set of skills.