Well, in the fanfiction, Naruto might act like Tobi for strategic reasons. He could be aiming to throw his enemies off balance. Since Tobi was such an enigma in the Naruto universe, acting like him would make it difficult for opponents to predict Naruto's moves. Additionally, Naruto might have discovered some hidden knowledge about Tobi that makes him believe that acting like Tobi is the key to achieving a greater good, like saving the village or the entire ninja world. He could also be influenced by some external force, like a curse or a magic spell that makes him take on Tobi - like behaviors.
Maybe he's on a secret mission. He could be trying to infiltrate an enemy group that Tobi was associated with. By acting like Tobi, he hopes to gain their trust and gather information.
One reason could be that Naruto has some sort of new power or ability that is related to Tobi. This connection makes him unconsciously or even deliberately start acting like Tobi. It could also be that Naruto has lost his memory and somehow believes he is Tobi, so he starts behaving accordingly. Another possibility is that he's trying to deceive someone really powerful who is afraid of Tobi, and acting like Tobi gives him an advantage in that situation.
Well, in this fanfiction, it could be that Harry Potter has come across some sort of ancient magic or artifact that has changed him. Maybe this magic has a connection to Tobi's essence or the type of power that Tobi wields. Additionally, he might be trying to protect his friends in a different way. Tobi has his own way of safeguarding what he holds dear, which might seem appealing to Harry in a difficult situation. Or, there could be a prophecy or a hidden message that he has misinterpreted, leading him to believe that acting like Tobi is the right path to take.
Maybe for a new plot twist. It makes the story different from the original Naruto storyline.
One reason could be a genetic quirk or some sort of chakra - related phenomenon. Maybe there was a hidden seal or something that was activated, causing Naruto to inherit not just Minato's looks but also his mannerisms. Another possibility is some kind of reincarnation or spiritual connection.
Common themes include the idea of changing the course of the Naruto story. With Naruto being related to Tobi in some way in these fanfictions, it can lead to different outcomes for the ninja world. For example, they could team up against a greater threat that wasn't in the original plot. There's also the theme of uncovering secrets, as Tobi has many secrets, and Naruto could be the key to revealing them all.
Yes, there are quite a few. You can start by looking on popular fanfiction websites like Fanfiction.net or Archive of Our Own. Just search for 'naruto is tobi' and you'll find a variety of stories with different plotlines and interpretations.
Maybe Naruto admires Kakashi's skills. Kakashi is a highly respected ninja, and Naruto might want to be as good as him. So he starts acting like him to learn from him.
Maybe Naruto is put in a situation where he feels that Hiruzen's way of handling things is the best option. For instance, if there is a major crisis in the village that requires a more traditional and experienced approach, Naruto might choose to act like Hiruzen. Also, he could be influenced by the stories he has heard about Hiruzen and wants to fill his shoes in a way.
One reason could be for strategic purposes. Itachi was a highly strategic ninja. So, in fanfiction, Naruto might act like him to outwit a particularly difficult enemy. For instance, when facing an enemy who is a master of deception, Naruto could adopt Itachi's calm and calculating demeanor to see through the enemy's tricks.
Definitely Kakashi. Since he has a connection to both Naruto and Tobi (Obito), his presence can add a lot of depth. He can be the one who tries to bridge the gap between Naruto and Tobi, or he could be a source of conflict as he has his own history with Tobi.
One interesting Naruto and Tobi fanfiction could be where Tobi reveals his true identity to Naruto earlier in the story. This would completely change the course of events and their relationship. Naruto might try to reason with Tobi, believing there is still good in him.