It could be that Fred was involved in some sort of prank that backfired and affected Hermione negatively. For example, if he was testing a new product from Weasley's Wizard Wheezes and it made Hermione look foolish in front of others. An irate Hermione would be a force to be reckoned with. Fred, realizing the magnitude of her anger, quickly decides to run. He might be thinking of ways to make it up to her as he runs, like coming up with a grand apology or a way to reverse the damage he's done.
Clearly, Fred and Hermione are the main characters here. Fred is at the center of the action as he is the one on the run. However, Hermione's anger is what drives the plot forward. Without her being irate, there would be no reason for Fred to run. So both characters play crucial roles in this fanfiction concept.
Well, in some versions, Fred might use his quick-witted magic skills. He could cast a distraction spell to draw Bellatrix's attention away from Hermione. Maybe a spell that creates multiple illusions of himself, confusing Bellatrix long enough for him to grab Hermione and disapparate them to safety.
Fred could use his knowledge of the Weasley family's magic tricks and inventions. He might have some secret gadget or spell that can break through the barriers or spells that are being used to torture Hermione.
Hermione, on the other hand, could become more daring. She is already brave, but this situation could push her to take risks she normally wouldn't. She might be the one to suggest trying untested spells or taking paths that are less known in order to escape. As they run, she could learn to trust her instincts more and not just rely on book knowledge.
Their relationship would likely become more serious and responsible. They'd have to put aside some of their youthful antics as they now have a child to think about, even if it's from the future. There would be a new level of trust and understanding as they work together to deal with the situation.
In the fanfiction, it could be that Ron is consumed by jealousy or a false belief. This causes him to lash out at Hermione in a way that endangers her. Fred, having a protective streak, especially towards his friends, jumps into action. He might engage Ron in a duel of sorts, not with the intention to harm but to subdue him long enough to break whatever spell or negative emotion has a hold on Ron and thus save Hermione.
Well, in this fanfiction, it might be that Ron has been misled by a malicious wizard who has poisoned his mind against Hermione. Fred, being observant, realizes what's going on. He uses his charm and wits to first distract Ron and then casts a spell to free Ron from the mind - control, all while protecting Hermione from Ron's erratic behavior.
One possible fanfiction could be where Fred and Hermione discover their Animagus forms together. Maybe they transform in the Forbidden Forest and have to rely on each other to get back to Hogwarts safely. Their forms could be something that represents their personalities, like Fred could be a fox, known for its cleverness and playfulness, and Hermione could be an owl, symbolizing wisdom.
Perhaps the child is on a quest to bring back a lost magic that was present in the past but will be crucial for the future. Fred and Hermione are initially skeptical but as they see the child's abilities and the evidence of the disappearing magic, they start to believe. Together, they embark on a journey to find ancient spells and artifacts that can restore the magic, all while dealing with the challenges of time travel and the secrets the child holds.