Yes, in some fanfictions Naruto is depicted as the Rikudo Sennin. Fanfiction writers often like to create alternate storylines where characters take on different roles or powers, and this is one such example.
One implication is the change in power level. Naruto as the Rikudo Sennin would be extremely powerful, which could lead to different battle outcomes. For example, he could easily defeat enemies that were previously tough for him.
The 'naruto heir of the rikudo sennin fanfiction' might be centered around Naruto's growth as the heir. It could include things like Naruto learning the secrets of the Rikudo Sennin, perhaps unlocking some hidden jutsu or knowledge. There could be a focus on the political implications as well, like how different ninja villages react to Naruto being the heir. And it may also involve Naruto's struggle to balance his newfound responsibilities with his own goals and values.
The main character is Naruto as he is the heir of the Rikudo Sennin. However, other characters are likely to be involved too. Jiraiya could be a key figure in this fanfiction, considering his connection to Naruto and his knowledge of the past. Additionally, the antagonists in the Naruto world might also be main characters in a sense, as they would oppose Naruto's rise as the heir, either out of jealousy or a desire for power themselves.
Wonder Woman is another character that might be paired with Itachi. Her Amazonian warrior heritage and Itachi's shinobi skills can be compared and contrasted. Their different views on justice, with Wonder Woman coming from a more idealistic place and Itachi having a more pragmatic approach, can lead to some great character - driven storylines in fanfiction.
Naruto would adapt to Itachi's life in several ways. Socially, he'd have to get used to Itachi's connections and rivalries. For instance, his relationship with Kisame would be different as Naruto. He might try to influence Kisame in a more positive way. Emotionally, he'd have to deal with Itachi's internal struggles. Itachi had a lot of guilt and secrets, and Naruto would have to either come to terms with them or find a way to resolve them. And in terms of combat, he'd practice Itachi's signature moves over and over to make sure he could use them effectively in battle.
They could develop a mentor - mentee relationship. Itachi uses his knowledge of the Sharingan to teach Naruto new combat techniques. As they train together, they bond over their shared experiences of loss and their desire to protect the people they care about. Naruto might also help Itachi come to terms with his past actions and find some peace within himself.
Itachi's personality could become more outwardly friendly. In the ninja world, he had to keep many secrets and be aloof. But in the wizarding world as Harry Potter, he might find it easier to let people in. He may also be more assertive in a different way. Instead of silently following orders like in his past life, he could take the lead in situations, using his own judgment based on his past experiences and newfound magical knowledge.