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.
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.
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.
Sometimes, other characters from the Harry Potter series can also play important roles in this type of fanfiction. For instance, George Weasley, Fred's twin, might be involved in helping Fred deal with the situation or providing comical relief. And Harry Potter himself could be there to support his friends through this strange turn of events.
Obviously, Fred and Hermione are the main characters. Fred, with his red hair and jovial personality, and Hermione, with her bushy hair and vast knowledge of magic. They are brought together by this baby project. It's likely that their different personalities, Fred's more laid - back and Hermione's more serious, will create interesting dynamics as they work on the project.
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.
It could be a jealous wizard who was once in love with Hermione. Discovering she has a child with Fred, this wizard might seek to harm the child out of spite. He could use dark magic to try and kidnap the child or cause trouble for the family.
One way to find good authors is to look at the reviews and ratings of different fred and hermione married fanfictions. Authors who get a lot of positive feedback for their character development, plot twists, and writing style are likely to be good. For example, an author who can make the relationship between Fred and Hermione seem believable and engaging is probably a great choice.
Draco Malfoy could be a likely antagonist. He might be jealous of the friendship or whatever relationship is developing between Fred and Hermione and try to cause trouble for them. Another could be some of the more strict professors who don't approve of their unorthodox ways of doing things.
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.