One key element is the injustice. Umbridge's act of whipping is a clear display of her tyrannical nature. It sets the stage for the twins' retaliation or their journey to resist her authority.
One popular 'Umbridge Whips Fred and George' fanfiction is 'The Rebellion Against Umbridge'. It shows how the twins cleverly outwit Umbridge after such a cruel act. The story has great character development and keeps you on the edge of your seat.
One possible consequence could be that she gets fired from her position. If Harry manages to expose her actions, the wizarding school or the Ministry of Magic might not tolerate such blatant abuse of power and take disciplinary action against her.
Another way Harry could respond is with a sense of shock and disbelief at first. Given that Umbridge is supposed to be in a position of authority, Harry might be momentarily stunned that she would go to such lengths. However, his Gryffindor courage would soon kick in, and he would likely start to resist, perhaps by using his magic in creative ways to protect himself or to disarm Umbridge, all while trying to uphold the values of the wizarding world that oppose such cruel and unjust treatment.
One reason could be that they played a particularly mischievous prank on her that went too far. For example, if they made her late for an important exam or meeting with a professor by using one of their wild magical inventions. Hermione, being a stickler for rules and academic achievement, might lose her temper and hex them in retaliation.
A characteristic is the playfulness. Since Fred and George are known for their pranks in the original series, this often carries over into the fanfiction. So, the kiss might happen in a really unexpected and fun situation. Like, they might be in the middle of a prank gone wrong or right and then suddenly share a kiss. Also, the fanfiction usually explores their individual personalities within the context of their relationship. For instance, Fred might be more impulsive when it comes to expressing his feelings, while George is more reserved. The kiss then becomes a way to show how they navigate these differences.
In some fanfictions, they could be seen as soulmates in a platonic sense. Their strong bonds of friendship and the adventures they share might be interpreted as a form of soulmate - like connection.
A popular one is 'Fred, George and Hermione: A Magical Bond'. It's a fanfiction that focuses on the friendship between them. It shows how they support each other through different challenges at Hogwarts, like dealing with exams and the occasional dark magic threats. The way the author portrays their bond growing over time is really well - done.
The theme of innovation is also quite common. Given their inventive nature in the original stories, fanfictions may show them coming up with new magical products or spells. Additionally, loyalty to their family and friends is a recurring theme. They are always there for those they care about in many fanfictions.