A cool plot could be Naruto being reincarnated as an enemy of Konoha in a parallel universe. He has all his memories but has to fight against his former friends and home. This would lead to a lot of internal conflict for him. Also, he could be reincarnated into a time where there are new and strange forms of chakra that he has to learn to master in order to save the world again. For example, a chakra that is a combination of nature and spiritual energy.
Well, in some fanfictions, Naruto could be reincarnated in a world where all the tailed beasts are humans. He has to find a way to convince them to work together again like they did in his past life. Or perhaps he is reincarnated into a future where technology has replaced ninjutsu, but he discovers that an ancient evil can only be defeated with ninjutsu, so he has to find a way to bring it back into the mainstream.
One idea could be that the reincarnated character has knowledge of future events in the Naruto world. For example, they know about the Akatsuki's true intentions from the start and can start preparing the Hidden Leaf Village earlier. Another plot could be that they have a special power from their past life that no one in the Naruto world has seen before, like the ability to control modern technology within the ninja world. A third idea is that they are reincarnated as a side character and their actions end up changing the entire course of the original Naruto story in unexpected ways.
A plot could also involve Naruto being reincarnated at a different point in the Naruto timeline. For example, if he was reincarnated during the time of the Sage of Six Paths. He could have a major impact on the early development of ninjutsu and the ninja world.
Perhaps the reincarnated character has a special ability that allows them to communicate with the unique monsters in the Tensura world. This ability could lead to forming alliances that no one else could, and they could use these to their advantage to gain power and protect the world from new threats.
One idea could be that the reincarnated character has memories of the future of the Boruto world. So they try to prevent certain disasters from happening, like the fall of Konoha. Another could be that they use their modern - day knowledge to revolutionize the ninja technology in the Boruto universe.
One idea could be that the reincarnated character has knowledge of future events in the Danmachi world. For example, they know about certain monsters' weaknesses or hidden treasures. This could give them an edge as they navigate the dungeon. Another could be that they are reincarnated as a non - human race in the Danmachi world, like a spirit or a dwarf, and they have to deal with the different cultural and physical aspects of that race. A third idea is that the character reincarnates with a special ability that doesn't exist in the normal Danmachi world, like the power to control time on a small scale, which would completely change their adventures in the dungeon.
One idea could be that the protagonist, reincarnated as a dinosaur, tries to prevent the extinction event. They could use their human knowledge to find a way to survive the impending asteroid or environmental changes.
A third plot idea could be the relationship between the sword and the other characters. Since it's reincarnated as a sword, it can't communicate in the traditional sense. So, how it forms bonds with the people around it, like the protagonist who might be the one it eventually chooses as its wielder. It could be through dreams or visions that it imparts knowledge or feelings to the characters, creating a unique and complex relationship dynamic.
One idea could be reincarnating as a vampire hunter who then falls in love with a vampire. Another could be being reincarnated as a long - lost relative of one of the main characters and causing chaos in the vampire - human relationships. Or perhaps being reincarnated as a wolf shifter with a special ability that changes the power dynamics in the Twilight world.
One idea could be the main character exploring a new, hidden part of the slime world that has unique magic and creatures. For example, a place filled with sentient plants that can communicate with slimes.
One plot idea could be that the reincarnated character has knowledge of future events in the Naruto story. They could use this knowledge to try and prevent tragedies, like the death of certain characters. Another idea is that they are reincarnated as a villain from their past life and have to struggle with their old evil tendencies while in the Naruto world. Or they could be reincarnated into a completely different body, like an animal, and have to find a way to become a ninja in that form.