A great plot idea could be an SI OC who has the ability to communicate with animals on a deep level, like a reverse Inuzuka. This OC could use the animals' knowledge of the land and hidden passages to gain an advantage. They could also form alliances with powerful animal summons, like the giant toads or snakes, but in a way that is different from the traditional summoners. This would create a whole new dynamic in the story as they navigate the politics and battles of the Naruto world.
A cool plot idea is an SI OC who is from a world where music has power. In Naruto, they discover that their musical abilities can either soothe or disrupt chakra. For example, they might be able to play a tune that calms down a rampaging bijuu. Another could be an OC that has the ability to see the past of a place just by touching the ground. This could lead to uncovering long - lost secrets about the founding of the ninja villages. Or perhaps an OC who can transform into different elements, not just the basic ones in the Naruto world, like turning into a magnetic field to disrupt enemy attacks.
To create a memorable OC, give them a unique appearance. For example, they could have heterochromatic eyes with different abilities tied to each color. Also, a distinct personality trait like being extremely loyal to a fault or having a hilarious sense of humor in the midst of serious situations in the Naruto world.
Another plot idea is that Naruto pretends to be insane to protect someone. For example, if there is a powerful enemy after a friend of his, he could act crazy and claim that he has some sort of prophetic madness that warns him of danger. This would make the enemy focus on him instead of his friend, while he figures out a way to defeat the foe.
Well, an OC in a 'crazy naruto si fanfiction' could be a character who is completely out of sync with the normal Naruto world values. Maybe they come from a modern world and their modern ideas seem crazy to the shinobi. They could introduce things like modern technology in a really strange and unexpected way, like using a smartphone as a weapon somehow. They might also have a crazy training regime that shocks everyone around them.
One idea could be Naruto somehow getting transported to the DC universe and joining the Justice League. He could use his ninja skills, like shadow clones, to confuse villains during battles. For example, when facing a powerful foe like Darkseid, Naruto's clones could swarm him, making it hard for him to target the real Naruto.
Another plot could revolve around an OC who is from a different time or dimension and has knowledge of advanced technology. They could use this knowledge to try and create new defenses against the Titans. Maybe they develop a weapon that doesn't require blades and 3D maneuver gear but is more effective in taking down Titans.
One idea could be an OC who has the power to manipulate time in a limited way. They could use this power to try and prevent certain tragedies in the Code Geass world, like the fall of certain regions. Another plot could involve an OC who is a master strategist from a different universe and has to adapt their skills to the complex politics of Code Geass. A third idea might be an OC who can communicate with machines, which could lead to some really unique interactions with the mechs in the series.
One idea could be that the reincarnated Madara retains some of his past memories but is in a completely different world. So he has to adapt to new cultures and powers while still having his old ambitions deep inside. Maybe he tries to find a way to bring back the old ninja ways in this new world.
Some cool insane Naruto crossover fanfiction concepts could involve a crossover with the Bleach universe. Naruto could interact with Soul Reapers and Hollows. He might even learn some Quincy techniques. Another possibility is a crossover with the Avatar: The Last Airbender world. Naruto could learn bending techniques and combine them with his ninja skills. Also, a crossover with the Godzilla universe would be wild, with Naruto having to face off against giant monsters.
One idea could be an OC who is a brilliant detective in Gotham, trying to solve cases alongside Batman but often having a different approach. For example, the OC might focus more on the psychological aspects of the criminals, while Batman is more into the physical evidence. Another idea is an OC who has a special power, like the ability to communicate with animals, which helps Batman in his fight against villains who use animals in their crimes. A third idea could be an OC who is from a different dimension and has a different set of values, creating interesting conflicts and collaborations with Batman as they try to understand each other.