One interesting plotline could be the twins getting involved in a magical mystery that Harry also stumbles upon. Maybe they discover a hidden spell together and have to figure out how to use it properly.
One idea could be that Harry has to team up with the Patil twins for a special magic project at Hogwarts. They discover a hidden magical artifact in the library and have to figure out how to use it without getting caught by the teachers.
One idea could be the Patil twins using their unique magical skills, perhaps learned from a different magical culture, to help Harry in a dangerous mission. For example, they might have a special way of divination that gives Harry an edge in predicting Voldemort's moves.
One idea could be a story where Harry Potter has to team up with the Patil twins for a special Hogwarts project. They might discover a hidden chamber together and have to solve various magical puzzles within it.
In some fanfictions, their twin status might change the way the Sorting Hat sorts them. Instead of Harry going to Gryffindor, perhaps they both end up in Hufflepuff because of their shared values that are more in line with that house. This could lead to a completely different story arc where they are known for their loyalty and hard work rather than Harry's typical Gryffindor - like heroics. Also, their family relationships would be different. The Dursleys might have to deal with two magical children instead of one, which could change the way Harry (and Neville) grow up and view the Muggle world.
One interesting plotline could be that they discover their twin relationship early on and try to keep it a secret from everyone at Hogwarts. They might secretly help each other out in classes and with their rivalries against other houses.
A possible plotline is their different magical abilities. Percy might be really good at transfiguration and charms, while Harry has his natural talent for defense against the dark arts. They could compete in a friendly way, but also learn from each other. For instance, Percy could teach Harry some advanced transfiguration spells, and Harry could show Percy some new defense techniques he learned from his experiences.
One interesting plotline could be the Carrow twins trying to manipulate Harry into joining their dark deeds, but Harry resists and tries to turn them to the good side. Another could be Harry forming an unexpected friendship with one of the twins that causes a rift between the twins themselves.
Perhaps a plotline where they form a special bond while facing a common enemy. They might have to work together to outwit a dark wizard or a tricky magical creature. It could involve using their unique skills and teamwork to overcome challenges.
An idea could be that the Patil twins are given a lemon by Professor Snape as part of a special assignment. Harry Potter helps them figure out what to do with it. Turns out, the lemon has a spell trapped inside it. When they break the lemon open, the spell is released and it takes them on a wild ride through different Hogwarts corridors that are usually locked or hidden.
It could start casually. Maybe they meet during a Quidditch match or a school event. They exchange glances and start small conversations. As time goes on, they find they have common interests like a love for a particular type of magic or a shared dislike for certain school rules. This builds a foundation for their relationship, which gradually deepens into love, leading to marriage.