Well, they could be reincarnated as a side character from the original Danmachi story. Let's say they become a member of a small and overlooked Familia. They then use their modern - world knowledge to improve the Familia's standing. Maybe they introduce new training methods or ways to manage the Familia's finances. Or perhaps they are reincarnated into the body of a character who was supposed to die soon in the original story. They then have to find a way to change their fate while also dealing with the complex relationships in the Danmachi world.
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.
A plot could revolve around a magic item that is found in the dungeon. This item has the power to grant a wish, but only one. The harem members all have different desires, which causes a lot of tension. Some might want to use it for Bell's benefit, while others have more self - centered wishes. This would lead to a complex web of emotions and decisions within the harem.
Maybe an OC that is an artificial being created by a powerful wizard long ago. The OC wakes up in the present day of the Danmachi world and has to learn about the new society and dungeon while also dealing with the fact that they are different from everyone else. They could form unique relationships with the existing characters as they try to find their place.
A plot for Gilgamesh Danmachi fanfiction might be that Gilgamesh discovers a hidden part of the dungeon that has a connection to his past. Maybe it contains ancient artifacts or secrets related to his origin. As he delves deeper, he has to team up with some of the local adventurers from Danmachi. They are initially skeptical of him but soon realize his power and knowledge are invaluable. Together, they face various challenges and traps, and Gilgamesh also has to deal with his own internal struggles as memories from his past start to resurface.
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 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.
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 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.