Madara Uchiha could be the cause. Even if he's dead, his influence could still be around. Maybe he left behind some sort of genjutsu or curse that affects Naruto's mind and drives him crazy. After all, Madara was always planning something big and Naruto would be a great pawn if he's out of control.
Well, one interesting aspect could be the unique character interpretations. Fanfic writers often explore different sides of Naruto characters that we might not see in the original series. For example, they might delve deeper into a character's backstory or give them new powers. Another aspect is the different plotlines. It can range from alternate universe scenarios, like what if Naruto was raised in a different village, to completely new adventures with the existing characters.
In a 'naruto goes crazy fanfic', Naruto might start to use his powers in unpredictable and extreme ways. He could go against his usual moral code and attack his friends or allies by mistake, perhaps due to some powerful dark influence or a misunderstanding of a new jutsu he's acquired.
Perhaps an OC that is a foreign ninja from a far - off land with a completely different fighting style based on music. They use sound waves to attack, defend, and even hypnotize opponents, much like how Jiraiya uses his toad - related jutsu in creative ways.
You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own (AO3). These platforms have a large collection of Naruto - related fanfics, including the 'crazy' ones. Just use the search function and type in 'naruto crazy fanfic' or related keywords like 'alternate Naruto story' or 'crazy Naruto adventure'.
Well, in many Naruto fanfics, the crazy strong aspect often involves unique jutsu combinations. For example, the protagonist might create a new type of Rasengan by combining it with a lightning - based chakra. Also, the character development can be really intense, with characters achieving power levels that are far beyond what we see in the original series in a very short time.
The main protagonist usually stands out. If it's a self - insert type of fanfic, that character might have some OP (over - powered) abilities right from the start. Another could be a reimagined version of Naruto himself, with even more powerful forms of his signature jutsu. And villains can also be very impressive, like a super - strong Akatsuki member who has some new and terrifying powers.
Well, one possibility could be that Naruto is exposed to some extremely powerful and corrupting jutsu that slowly drives him insane. Maybe he starts seeing hallucinations of his friends turning against him and that messes with his mind big time.
The characters might include some lesser - known characters from the Naruto series who get a chance to shine. For instance, Rock Lee could be part of the crazy team, teaching his unique taijutsu training methods to the others. And then there could be a character who is a foreign ninja, bringing in new techniques and a different perspective to the training.
The plot could involve a group of underdog ninjas forming a crazy team. They might start with really wacky and unorthodox training methods, like training on a constantly shifting terrain made by a jutsu. Their goal could be to master a unique and powerful ninja skill that no one has ever seen before in the Naruto universe.