Another plot could be that the harem members are all from different dimensions. Seiya has to find a way to keep them together and safe in his own world. He is cautious about introducing them to his world's customs and potential dangers, while also dealing with his own growing feelings for each of them.
One idea could be that Seiya has to protect his harem members from a new, powerful enemy. Each member of the harem has a unique skill that can help in the fight, but Seiya is overly cautious and tries to keep them out of harm's way as much as possible.
Perhaps in the fanfic, Seiya accidentally creates a love triangle within his harem. For example, two of the harem members develop feelings for each other as well as for Seiya. This could lead to a lot of comical and tense situations as Seiya tries to navigate through their emotions.
Ristarte could be one of the main members. She has a strong connection with Seiya from the start of the story. Her kind-hearted and sometimes naive nature could make for an interesting dynamic within the harem.
Common themes in Seiya Vakierye fanfics include his unique combat style which is a result of his caution. This can lead to some really creative battle scenes in fanfics. Additionally, there's the theme of his relationship with the divine beings in the 'Cautious Hero' universe. How he deals with their expectations while still being true to his cautious self is a rich area for fanfic exploration. And, of course, the theme of self - discovery as he navigates his role as a hero in this strange world.
There are a lot of factors that can make a 'Cautious Hero' Seiya Vakierye fanfic great. It could be how well the author captures Seiya's overly cautious nature while still making him a likable hero. A good fanfic might also explore his relationships with other characters in new and interesting ways. For example, it could delve deeper into his interactions with the goddess he serves. To find such fanfics, you could also check out Tumblr. There are often fanfic reviews and recs on there.
For an 'accidental hero fanfic', think about a situation where a character doesn't mean to be heroic but circumstances make them so. A gardener could be watering plants when he discovers a strange object buried in the soil. It turns out to be an ancient artifact with magical powers that he unwittingly uses to save his town from a natural disaster. Or perhaps a taxi driver who picks up a passenger who is actually a wanted criminal on the run. The driver unknowingly takes a wrong turn that leads to the criminal being caught by the police. Another idea is a waitress in a diner who overhears a conversation between two villains planning a bank robbery. She accidentally spills coffee on them, causing a commotion that alerts the authorities and thwarts the robbery.
One idea could be to give the hero a unique power that is not typically seen in the original story, like the ability to communicate with animals. This would add a new dynamic to the character and open up different storylines, such as using the animals to gather information or help in battles.
One idea could be that the main character, a new fighter in the BlazBlue universe, suddenly finds himself surrounded by various female characters who are all intrigued by his unique fighting style. They start to vie for his attention, leading to comical and heartwarming situations.
One idea could be that Aladdin, after becoming a prince, has to deal with various princesses from different lands who are all attracted to him for different reasons. Maybe some are charmed by his heroic deeds, others by his newfound wealth and status. Another plot could be that the genie accidentally creates a situation where multiple female characters from Agrabah develop feelings for Aladdin at the same time, leading to a series of comical and romantic misunderstandings.
Perhaps in this genderbend harem fanfic, the female Deku is a new transfer student to a school dominated by male students who are all attracted to her kind - hearted and heroic nature. And they all compete to be her sidekick in battles.
Perhaps the OC has a hidden past related to a villain organization, and the harem members are initially suspicious but as they get to know the OC better, they realize there's more to the story. This could create a lot of drama and conflict as the OC tries to prove their loyalty while also dealing with their growing feelings for the harem members.
One idea could be to explore the backstory of a minor character. For example, what was the life of a random student in Class 1 - B like before coming to U.A. High? Another could be a 'what if' scenario, like what if Izuku Midoriya didn't get One For All. Maybe he'd develop his own unique quirk through sheer determination and hard work. Or a fanfic could focus on the villains' side, showing how they came to be and their motives in more depth.