Another possibility is an old enemy of Klaus. They might see this situation as an opportunity to get revenge on Klaus. They could try to use the OC's pregnancy to manipulate Klaus or cause trouble between him and the OC.
Well, one possible plot could be that Klaus and the original character (OC) have an unexpected encounter. Maybe they start off as friends or even enemies at first. Then, through a series of events, like a shared adventure or some sort of emotional connection, they end up getting closer. And after a passionate moment, the OC discovers she's pregnant. This could lead to a whole bunch of drama, like Klaus trying to figure out his role as a potential father, and the OC dealing with the social and personal implications of being pregnant out of a rather unconventional relationship.
The antagonist could be an evil wizard. He might be jealous of the oc's natural magical abilities and decides to turn the oc into a kitten to weaken them. Another option is a vengeful fairy. Maybe the oc accidentally stepped on her flower garden, and in retaliation, she uses her magic to transform the oc into a kitten. Or it could be a power - hungry sorceress who wants to use the oc's new kitten form for some dark magic experiment.
Well, one possible plot could be that Klaus and Elena initially have a very tumultuous relationship. Maybe they are forced to work together against a common enemy and during that time, their feelings change. Klaus might become more protective of Elena, and this newfound closeness leads to her getting pregnant. It could then explore how they deal with the pregnancy in a world full of supernatural drama and danger.
It could lead to a whole new dynamic in the story. Maybe it would bring them closer or cause a lot of drama and complications.
The witches in the story can also be antagonists. They may have their own motives and prophecies related to the baby. They could try to manipulate the situation to their advantage, perhaps by using dark magic to influence the OC or the baby's fate.
Another option is a group of anti - magic fanatics. They view Percy's wings as an abomination because they are a result of magic. They hunt Percy down, believing that by getting rid of him and his wings, they are protecting the world from the 'evils' of magic.
It could still be Hawk Moth. Since Nathalie has been working with him, getting a Miraculous might make her a threat to his plans, so he could turn against her and become the main antagonist.
It could be a long - lost relative of an old enemy. For example, if there was a powerful dark wizard that Natsu defeated in the past, this relative might come seeking revenge and be powerful enough to kill Natsu.
Maybe an evil artist from another dimension. This artist is jealous of Nathanael's pure creativity and the unique power of his miraculous. They come to Nathanael's world to steal his ideas and use his miraculous for their own dark purposes, like creating chaos and destruction through art.
Maybe the main antagonist is an old enemy of Robin's who becomes jealous of his new powers. This enemy then devises a plan to steal Robin's powers or at least neutralize them so that he can regain his former status as the top threat. He could be a powerful martial artist or a criminal mastermind with a large network of henchmen.