It could be one of the regular bullies at her school. They might target Louise because of her feisty nature. They see her as a threat to their dominance in the schoolyard and decide to teach her a lesson. However, they don't realize the consequences of their actions and end up really hurting her.
One possible plot could be that Louise gets hurt during one of her pranks. Maybe she miscalculates and falls from a height while trying to set up a fake spider to scare someone. Her family then has to rally around her. Tina might be really worried and try to write her a heartfelt poem to cheer her up. Gene could offer to play her favorite songs on his keyboard at her bedside.
Often, bobs burgers louise fanfic focuses on Louise's rebellious nature. Writers like to explore how she gets into and out of trouble. For example, she might start a prank war at school.
There's also a chance that an old enemy of the Thundermans, who has been lying low, resurfaces and decides to take revenge on the family by hurting Max. This enemy might have some new powers or a new plan that catches Max off - guard and leads to his injury.
A supernatural hunter might be responsible. Hunters in Teen Wolf sometimes misjudge humans as being involved with the supernatural. They could have thought Danny was a part of something bad and injured him while trying to 'deal' with the situation.
There's also a fanfic called 'Louise and the Schoolyard Rival'. In this story, Louise has to deal with a new kid at school who is trying to take over her role as the toughest in the playground. It's a really engaging read as it shows Louise's insecurities and how she overcomes them in her own unique way.
It could be an enemy dragon. There are many rival dragons in the Fairy Tail universe, and if they see Natsu with Igneel, they might target Natsu to get to Igneel. Dragons can be very territorial and protective of their own, and an enemy dragon may see Natsu as a weak point to attack Igneel through.
It could be a new, inexperienced doctor. Maybe they make a mistake during a surgery that Meredith is involved in, accidentally causing her harm. For example, they might misread a vital sign or use the wrong instrument.
It could be a rival assassin. They might see Kassandra as a threat to their own reputation or contracts.
It could be an old enemy like Luke. He has always had a vendetta against Percy and would not hesitate to cause him harm. Maybe Luke has some new evil plan and hurting Percy is part of it.
Another possibility is a jealous student at Hogwarts. There are always students who might be envious of Hermione's academic achievements. They could set up a prank or a trap that goes wrong and ends up hurting her. For example, a student who is tired of Hermione always getting the top grades in Potions might tamper with her cauldron during a class experiment, resulting in an explosion that hurts her.