Another plot could involve Gasper trying to help his harem members improve their skills. For example, he could train with them in combat or help them study for school exams, which would strengthen their relationships. He could also introduce his harem to his family in the supernatural world, leading to some interesting cultural exchanges and perhaps some initial misunderstandings that they would have to overcome.
One idea could be Gasper using his unique powers to protect his harem members from a new threat in the high school. Maybe a group of rogue devils tries to target the girls he cares about.
A more complex plot could be centered around a prophecy that involves Gasper and his harem. The prophecy might say that they are the key to saving the high school from a great evil. This would force them to work together, discover new powers, and face their own insecurities along the way.
A really interesting plot could be Gasper getting transported to a different dimension or a parallel world within the DxD universe. In this new place, he has to rely on his wits and the basic powers he already has to survive and find a way back home. Along the way, he meets different versions of the characters he knows, and through his interactions, he learns more about himself and his place in the grand scheme of the DxD world.
One plot idea could be Asia and Gasper teaming up to solve a mystery within the school. Maybe there are strange disappearances and they use their unique abilities to figure out what's going on. Another idea is a story where they go on a journey to find a magical artifact that can help protect their friends. And a third could be a slice - of - life story where they help each other deal with personal problems like Gasper's shyness and Asia's insecurities about her new life in the supernatural world.
Maybe the story could revolve around the main character accidentally hypnotizing the wrong person in the harem, which leads to a series of comical and challenging situations. He has to then find a way to reverse the effects without causing too much chaos among the group.
Well, a plot could revolve around the main character getting sent on a special mission by the school's authorities. Along the way, he meets new female characters who end up joining his harem. Also, there could be a plot where the harem members start developing their own unique powers and they have to train together with the main character. This could strengthen their bonds not only as a harem but also as a team. And then, there could be a mystery element where someone is secretly trying to break up the harem, and the main character has to find out who it is.
The OC could be a new transfer student with a mysterious past. As the story progresses, it's revealed that they are on a quest to find a powerful artifact that is somehow connected to the characters in the harem. This could lead to adventures together, and the building of deeper relationships within the harem.
Perhaps a plot where the members of Vali's harem start developing new and unique powers. They then have to train together to control these powers while also facing threats from other factions in the Highschool DxD universe. This would involve a lot of character development and relationship building within the harem.
One idea could be an OC who is a transfer student with a unique power. The harem could form as different characters in the Highschool DxD world are intrigued by this power and gradually fall for the OC. For example, the OC might have the ability to heal wounds instantly, which is very useful in the battles that often occur in the DxD world.
One idea could be the main character, as the harem queen, having to deal with rivalries among the members of the harem. Maybe there's a new girl who challenges her position and she has to prove herself through various magical trials.
One idea could be an OC who is a transfer student with a unique power, like the ability to manipulate time slightly. This catches the attention of the main characters in the Highschool DxD world. The harem could start to form as different characters are intrigued by the OC's power and mysterious background.
A good concept for 'highschool dxd harem fanfiction' could be a time - travel story. The harem and Issei could be sent back in time and have to navigate the past while also dealing with their feelings for each other. They might meet younger versions of themselves or their predecessors. Another idea is to have a mystery element, where something strange is happening in the school and the harem has to solve it together. This could bring out different sides of each character and also develop their relationships further.