Maybe in the fanfiction, Katsuki has always been overconfident and underestimated his opponents. The bad karma could be that he faces a powerful enemy who completely outmatches him. He then has to struggle not only with the physical threat but also with the psychological blow of his own hubris. This would force him to re - evaluate himself and his actions.
One could be Izuku. He might be there to witness Katsuki's downfall due to bad karma. Izuku could be the one who tries to help Katsuki see the error of his ways, even though Katsuki has been unkind to him in the past. It would add an interesting dynamic to the story.
Well, he could be at a skating event's after - party. He accidentally drinks too much champagne. As he gets drunk, he might mistake a famous skater for his coach and start asking for advice on his next routine. This would lead to some comical misunderstandings.
The plot could involve Yuri being hypnotized to overcome his stage fright. Maybe a well - meaning friend or a new coach discovers that Yuri has deep - seated fears that are affecting his performance. They use hypnosis to take him into a state where he can face those fears head - on. In this trance - like state, he relives his worst skating moments but with new confidence, and when he comes out of it, he's a changed skater, ready to take on any competition without hesitation.
One possible reason could be if Katsuki is rude to Izuku. Since Inko is Izuku's mother, she would naturally get mad at someone who is mean to her son. For example, if in the fanfiction Katsuki makes fun of Izuku's dreams or abilities in a particularly cruel way in front of Inko, she would likely become angry.
Katsuki could become more empathetic. Since One for All is a power that has a lot of history and meaning, he might start to understand others better. For example, he may understand Deku's journey more deeply. He'll still be his bold self, but with a new layer of understanding and compassion added to his personality.
Another plot idea could be a time - travel or alternate universe scenario. They get married in a different world where the events of 'Assassination Classroom' happened differently. For instance, they might be in a world where the threat to Earth from the moon was resolved in a different way, and their marriage is part of the new world order. They would then have to navigate this new world while being a married couple.
In some fanfictions, Katsuki gets One for All and it completely changes his relationship with Deku. Instead of rivalry, they start to train together more closely. There are also fanfictions where Katsuki uses One for All to save the day in a situation where no one else could. For example, when facing a powerful new villain that the other heroes can't handle. He might combine his natural combat skills with the power of One for All in a unique way.
The fanfiction could be about the adventures Stiles and the puppy have. They could go on hikes together in the forest near his town. During these hikes, they might encounter strange things or other supernatural beings. The puppy could either be scared or curiously investigate. Stiles would have to protect the puppy and also figure out how to deal with whatever they come across.
Well, perhaps in this fanfiction, Percy gets wings through some magical event. Maybe he makes a deal with a powerful magical being who gives him wings as a reward or a means to achieve a greater purpose. It could also be that he inherits the wings due to some special bloodline or heritage that he discovers.
In a 'naruto gets pregnant fanfiction', perhaps there is a powerful enemy who casts a spell on Naruto. This spell not only endangers his life but also has the side - effect of making him pregnant. The story could then revolve around Naruto trying to find a way to reverse this while also dealing with the reactions of his friends and the village.