Well, a plotline might involve a bet about who can better handle a dangerous magical creature encounter. Hermione would rely on her extensive book knowledge, while Fred would use his hands - on experience with magical creatures from his time at the Burrow. It could also be a bet on who can win over a particularly stubborn Hogwarts professor to their side for a school project. Maybe a bet on who can brew the most effective potion in a limited time too.
One common plotline is them teaming up to play pranks on other students in Hogwarts in a more elaborate and creative way. Since Hermione is very intelligent and Fred and George are creative with their pranks, they combine their skills.
Well, a plotline could be that Fred's crush on Hermione creates some tension within the Weasley family. His siblings might notice his change in behavior around her and start teasing him or trying to play matchmaker. Meanwhile, Hermione could be completely oblivious to Fred's feelings at first. She's so focused on her studies that she doesn't pick up on his subtle hints, like the way he always offers her the last piece of treacle tart. This could lead to some comical misunderstandings as Fred tries harder and harder to get her attention.
One possible plotline could be that Hermione discovers a dangerous curse about to befall Fred during a battle and uses her quick thinking and vast magical knowledge to counter it. For example, she might create a complex protective charm just in time. Another could be that Fred is trapped in a magical maze or trap, and Hermione uses her logic and spell - casting skills to break him free. Maybe she has to solve a series of riddles to get to him first.
In some fanfictions, Fred could be the voice of reason. He tries to talk Ron out of his actions when he sees Ron straying from Hermione. But Ron doesn't listen, and Fred has to deal with the fallout. There could also be a plot where Fred is in a relationship himself, and seeing Ron's behavior makes him reflect on his own relationship and how to be a better partner, while also trying to help Hermione through the difficult time.
Another plot could involve them facing challenges from the wizarding community. Some might not approve of their union because of their different backgrounds. They would have to fight against prejudice and prove that their love is strong enough to overcome any external difficulties. They could work together to show everyone that their marriage is based on mutual understanding and deep affection.
Hermione's perfectionist streak would show in a bet. If they bet on creating a magical item, she'd strive for it to be flawless. Fred, with his more creative and unorthodox approach, might come up with something completely unexpected. When it comes to communication during the bet, Hermione would be very precise and detail - oriented, while Fred would be more likely to joke around and be less formal. Their different values, with Hermione valuing knowledge and Fred valuing fun and adventure, can also be highlighted through the bet and how they approach winning it.
One possible plotline could be that they got engaged during a dangerous adventure. Maybe they were trapped somewhere and made a pact to get engaged if they survived. And then they keep it a secret because they know their friends and families might not approve. For example, Ron might be really shocked and angry at first if he found out.
One interesting plot could be the challenges they face in a world where a marriage law forces them together. Maybe they start off disliking each other but gradually find common ground through their shared love for magic experiments. For example, Hermione's organized approach and Fred's creative chaos in the lab could lead to some hilarious and heartwarming moments as they learn to work together in their new 'married' situation.
One idea is that they use their Animagus forms to solve a mystery in Hogwarts. For instance, they suspect there's a hidden chamber and as their animal forms, they can access places regular students can't. Fred might be a squirrel, able to climb into small holes, and Hermione as a cat, able to prowl around quietly.
One interesting scenario could be Fred's humorous attempts at teaching parenting skills in the class, which constantly make Hermione laugh. For example, he might use his Weasley's Wizard Wheezes products as analogies for handling different child behaviors.