Perhaps the OC is a transfer student from a different village. Gaara is intrigued by her different ways and culture. They start to study together about each other's backgrounds. As they do this, they fall in love. The high school setting could add elements like jealous classmates or strict teachers that try to keep them apart, but they overcome these obstacles.
One idea could be that Naruto and Gaara are paired for a school project. Naruto's outgoing nature clashes with Gaara's more reserved one at first, but as they work together, they discover they have more in common than they thought, like their love for their families. They end up becoming good friends and helping each other through high school challenges.
One plot idea could be that the OC is a powerful ninja from a hidden village. She meets Sasuke first during a mission where they initially clash due to Sasuke's brooding nature and her more optimistic outlook. Later, she also encounters Gaara. Gaara is intrigued by her because she doesn't fear him like others. The OC then finds herself in a love triangle, with Sasuke being drawn to her because she challenges him, and Gaara seeing in her a chance for a different kind of connection. As the story progresses, she has to choose between them while also dealing with the political unrest in the ninja world that affects all three of them.
Well, perhaps the OC is a musician who plays at a local café where Gaara often goes to read. Their music catches Gaara's attention, and they start to have conversations about life and dreams. Or the OC could be a volunteer at an animal shelter and Gaara comes in looking to adopt. They bond over their love for animals and that could be the start of their relationship.
One plot idea could be that Hinata and Gaara are assigned as partners for a school project. Hinata is initially shy around Gaara, but as they work together, they discover they have a lot in common. For example, they both have a love for art, but express it in different ways. Hinata through her delicate flower drawings and Gaara through his unique sand sculptures. As they get to know each other better, a friendship forms which could potentially turn into something more romantic.
Perhaps in a high school where there are different cliques. Gaara is from the more solitary group and Hinata is in the shy bookworm group. There's a school project that requires students from different groups to work together. They initially struggle but then find common ground in their love for gardening in the schoolyard. This shared interest blossoms into a deeper connection.
A possible plot is that the OC is a misfit who doesn't fit into the typical categories in the High School DxD world. They might be a human with no obvious supernatural powers at first, but through their wits and determination, they manage to form alliances with different factions. They could use their knowledge of human technology and psychology to gain an edge, and along the way, they develop a unique power or ability that no one expected.
One idea could be an OC who is a new transfer student at the high school in the Bleach universe. They have a hidden power that starts to surface when they encounter Hollows on school grounds. Another could be an OC who forms an unlikely friendship with a Soul Reaper in disguise at the high school and together they face off against a powerful Menos Grande that has targeted the school.
One idea could be an OC who is a gamer in the High School DxD world and uses their gaming knowledge to strategize in battles. For example, they might use their understanding of RPG leveling systems to level up their own powers faster than others.
Maybe in this high school fanfic, Gaara is a new transfer student and Sakura is assigned to show him around. Sakura, being the friendly girl she is, tries to break through Gaara's shell. As they explore the school together, they encounter various school cliques and challenges that bring them closer. For example, they might have to deal with a mean group of students who pick on Gaara for being different, and Sakura stands up for him, starting a beautiful bond between them.
Well, a plot could involve Takano being a new student at Ritsu's high school. Ritsu is assigned to show him around. At first, they don't get along because of their different personalities. But then, they discover a shared love for a particular subject or club in the school. This common interest becomes the foundation for their relationship to develop in the fanfiction.