One idea could be an OC who has a unique breathing style that combines elements from different existing styles. For example, a mix of water and flower breathing, which could create beautiful yet powerful attacks. Another could be an OC who is a demon but has retained some of their humanity and tries to find a way to coexist with the Demon Slayers. Maybe they form an unlikely alliance with a slayer to fight against more evil demons.
An interesting plot could involve an OC who is a master swordsmith but discovers a new type of ore that can enhance swords in a way never seen before. This leads to the OC getting involved with the Demon Slayers as they seek to use these enhanced swords against demons. Additionally, the OC could have a personal vendetta against a demon who destroyed their village in the past, which further fuels their motivation to help the slayers.
Well, your OC could be a foreigner who comes to Japan with a completely new form of combat that is not based on the traditional breathing styles. This OC might initially be seen as an oddity by the Demon Slayers but later prove to be a valuable asset. Or perhaps an OC that was experimented on by demons in the past and now has some strange powers as a result, and they use these powers to seek revenge on the demons who wronged them.
One idea could be a male OC who has a unique breathing technique passed down in his family. For example, a 'Star Breathing' technique that allows him to move as swiftly as a shooting star and his attacks are like constellations. Another plot could be that the male OC is a demon slayer from a foreign land, bringing in different fighting styles and cultural elements to the Demon Slayer Corps. Or perhaps he has a special bond with a demon that he is trying to redeem instead of killing.
One idea could be that the male OC has a unique power related to sound. He can use different frequencies to either paralyze demons or disrupt their attacks. For example, in a battle against a powerful demon in a forest, he uses the natural echo to amplify his power and gain an upper hand.
A great plot idea could be that Nezuko develops a new power that only works when she and Tanjiro are in close proximity. This power could be crucial in fighting a powerful upper - rank demon. For example, she might be able to create a protective shield around them both. Another could be a fanfiction where Tanjiro and Nezuko are invited to a demon slayer gathering in a far - off land. There, they meet other slayers from different regions and learn new techniques. Along the way, they also have to deal with some demons who are trying to infiltrate the gathering.
Another plot could involve a male OC who is a vampire hunter from a long line of hunters. But he has a different approach compared to Buffy. He believes in using more high - tech gadgets along with his physical combat skills. He teams up with Buffy and the Scooby Gang, and there are some clashes at first due to their different methods, but eventually they work well together.
An OC demon with the ability to communicate with animals could be an interesting concept. In the Highschool DxD setting, this power could be used in various ways. For instance, they could gather information from the animals in the school's forest area about the movements of enemies. Or they could use animals to deliver secret messages between different groups without being detected by the more high - tech surveillance methods used by some of the factions.
Well, he might travel back in time and end up being mistaken for a demon himself because of his unique ice powers. This could lead to all sorts of misunderstandings and battles. Or perhaps he travels back to the time of the first demon slayers and has to train with them using his modern - day knowledge of fighting demons. He could also travel to a time when a major demon - slayer organization was in its infancy and help shape it into a more effective force against demons.
One idea could be that the Demon Slayers are brought into the world of My Hero Academia. Tanjiro could learn about quirks and try to figure out how to use his skills in this new environment. Maybe some of the demons also gain quirks which makes them even more dangerous.
One idea could be an OC who is a witch from a long - lost coven that has a unique power related to time manipulation. This OC could accidentally get involved in Buffy's battles when trying to correct a time - related mishap in the supernatural world. Another idea is an OC who is a half - vampire, half - human hybrid, struggling to find their place between the two worlds. They could initially be seen as an enemy by Buffy but later become an ally.
Well, you could have an OC who is a tech - savvy dragon slayer. They create all sorts of gadgets and gizmos to help them in their dragon - slaying quests. For instance, they might have a drone that can scout out dragon lairs from a distance. Their backstory could involve a tragic event where a dragon attacked their village, and they vowed revenge. As for their appearance, they could have a cool, steampunk - like style with goggles and mechanical parts on their clothing. This would make them stand out in the RWBY universe. They could also team up with some of the main RWBY characters, teaching them about dragon - slaying techniques and learning from the Huntsmen and Huntresses about fighting Grimm.
For a Demon Slayer crossover fanfic, consider a crossover with Fairy Tail. The mages of Fairy Tail could use their magic to fight alongside the Demon Slayers. Natsu's fire magic would be really effective against some demons. Also, a crossover with Bleach would be fascinating. Soul Reapers could interact with the Demon Slayers, and they could exchange knowledge about dealing with spirits and demons. Ichigo's powerful sword skills could be a great addition to the fight against demons. And how about a crossover with Pokémon? Trainers could bring their Pokémon to the Demon Slayer world. A fire - type Pokémon like Charizard could be a powerful ally in battles against demons.