Well, a plot might involve a magical artifact. Hermione discovers it first, but Blaise, being possessive, wants to be the only one associated with it through Hermione. So he tries to control her access to it and the research around it. Or perhaps there's a magical competition, and Hermione is the star contestant. Blaise gets possessive, fearing she'll get too close to the other competitors, so he acts as her self - appointed protector throughout the event.
A possible plot is that Hermione starts tutoring Blaise. As they spend more time together, Blaise develops possessive feelings. He doesn't like it when Hermione spends time with her other friends. He tries to monopolize her time by creating study sessions that are really just an excuse to keep her to himself. Meanwhile, Hermione is oblivious at first to his possessiveness and just thinks he's really into the studies.
A third idea is that Blaise and Hermione are chosen to represent Hogwarts in an inter - magical school competition. They have to train together and overcome their differences. As they face challenges in the competition, they realize their mutual respect and admiration for each other's skills, which gradually turns into a romantic relationship.
If there's a situation where Hermione is in danger, Blaise saves her. After that, he becomes extremely possessive, believing that he has a special claim over her life. He starts to interfere in her daily activities, like who she can talk to and where she can go. Hermione may resist at first but gradually find herself falling for his protective, albeit possessive, nature. For example, he might stop her from going on a dangerous magical adventure with Harry and Ron because he's so possessive of her safety.
One idea could be that Blaise Zabini's dad was a close friend of Sirius Black in their youth. When Harry Potter starts to explore Sirius's past, he stumbles upon Zabini's dad and uncovers a story full of friendship, betrayal, and a long - lost magic that could change the course of the wizarding world again.
One idea could be that Blaise Zabini's dad was a secret agent for the Order of the Phoenix all along. He could have been spying on the Death Eaters from within their ranks and using his family's reputation to gain their trust. This would add a whole new layer of mystery and intrigue to the story.
A plot could be set post - Hogwarts. Draco and Blaise are trying to redeem their families' reputations in the wizarding world. Hermione, being the brilliant witch she is, is working on some important magical research. Their paths cross when a dark magic artifact resurfaces. Hermione believes it can be used for good if studied properly, while Draco and Blaise are worried it will fall into the wrong hands again. So they team up to protect it and study it.
One idea could be that Hermione is turned into a vampire against her will and Blaise, who has been a vampire for a long time, takes it upon himself to teach her how to control her new powers. He could introduce her to the vampire society and they end up falling in love during the process.
One plot idea could be that Draco and Blaise kidnap Hermione as part of a misguided attempt to gain some sort of leverage in the wizarding war. Maybe they think they can use her knowledge to find a way to end the conflict on their terms. Another could be that they believe she has a hidden power or secret that they want to uncover for their own gain. For example, they might think she has a connection to a powerful magical artifact that they are desperate to get their hands on.
A possible plot for this fanfiction could be that Blaise Zabini's dad was a double agent during the first wizarding war. Harry finds out about this when he starts to investigate some strange occurrences at Hogwarts that seem related to the past. This discovery leads to a series of events where Harry has to trust Blaise, despite their differences, to uncover the truth. Along the way, they might also learn more about their own families and the secrets they have been hiding.
One idea could be that Draco and Zabini kidnap Hermione for some sort of political gain in the wizarding world. Maybe they think her knowledge or influence can be used to their advantage. For example, they could be part of a group that wants to change the power structure at Hogwarts and believe Hermione holds the key to making it happen.
One plot idea could be that Hermione becomes overly possessive when Harry starts getting a lot of attention from other witches at Hogwarts. She might start using her intelligence to outwit those girls in a rather devious but humorous way, like tricking them into embarrassing situations so they'll stay away from Harry.