One possibility is that Umbridge, despite her cruel nature, sees some potential in Harry for her own gain. Maybe she tries to mold him into a perfect little wizard according to her strict and often wrong ideas. She could isolate Harry from his friends and force him to follow her rules blindly, which would create a lot of internal conflict for Harry as he tries to resist her brainwashing while living under her roof.
Harry's character would face a huge transformation. He'd be constantly on edge, having to deal with Umbridge's strict rules and her mean - spirited nature. This would make him more empathetic towards those who are oppressed. He'd also become more resourceful, finding ways to communicate with his old friends and allies despite Umbridge's attempts to isolate him. His experience with Umbridge would be a crucible that forges him into an even more resilient and determined hero, ready to take on not only Voldemort but also the corrupt elements within the wizarding establishment.
It's highly inappropriate. The relationship between Umbridge and Harry Potter is one of oppressor and oppressed in the original story. Adding such a violent and inappropriate act like spanking in a fanfic not only distorts the characters' relationship but also may have a negative impact on fans, especially younger fans who look up to the series for positive values and role models.
Fred and George Weasley are also favorites. Their mischievous nature can be amplified in fanfictions. They could create new and even more spectacular magical pranks specifically aimed at Umbridge, like turning her office into a never - ending maze of trick stairs and hidden rooms filled with various magical creatures that drive her crazy.
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.
She would probably be outraged. Umbridge is a strict and narrow - minded character in the original series. Fanfics often deviate from the 'proper' version of events she might think, and she'd likely see it as a distortion of the wizarding world she believes she upholds.
In the original series, Harry doesn't kill Umbridge. So this fanfic takes a very different turn right there. In the original, Umbridge is eventually removed from Hogwarts in a more traditional way, through the combined efforts of the students and the return of Dumbledore. But in the fanfic, Harry taking her life is a huge deviation from the non - violent and more rule - abiding nature of the main character in the original series.
It reflects it by exaggerating the already existing tension between Umbridge and Harry. In the original story, Umbridge was a very strict and unjust authority figure towards Harry. The fanfic just takes it further. For example, the cruel punishments she gives Harry in the books are made even more extreme in the fanfic.
In the context of the original Harry Potter story, it seems quite abnormal as Umbridge is a very unlikable character to Harry. But in the world of fanfic, anything can happen. Fanfic allows for different interpretations and character developments, so while it's not normal in the traditional sense, it's possible within the creative space of fan - created stories.
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.