One plot idea could be Kai and Klaus teaming up to take on a common enemy. Maybe a powerful witch or vampire that threatens their families or their own existence. They could start off as reluctant allies, but over time develop a grudging respect for each other.
A fanfiction could explore what would happen if Kai was brought into Klaus's family drama. For instance, how he would interact with Klaus's siblings like Elijah and Rebekah. Kai's chaotic nature might stir up new conflicts and alliances within the Mikaelson family. And perhaps Kai could learn some lessons about family loyalty from the Mikaelsons.
Another idea is a time - travel based fanfiction. Kai, with his knowledge of magic, accidentally sends himself and Klaus back in time. There, they have to navigate the past, deal with the versions of their families from that era, and find a way back to their own time while also not changing the course of history too much.
Well, a plot could revolve around Hope having visions of Klaus's future. In these visions, she sees that he's in great danger from an ancient evil that is about to be unleashed. So, she sets out on a journey to find a way to prevent this from happening. Along the way, they have to face their own personal demons as well. Maybe Hope has to come to terms with her anger towards Klaus for not being there for her all the time. Another plot could be that Klaus is cursed and Hope is the only one who can break the curse, but it comes at a great cost to her powers.
A great plot could be Klaus and the reader trying to find a safe haven for their unborn child. They might travel to different places, encountering various supernatural beings along the way. Some might offer help, while others might see the baby as a threat. Also, the internal conflict within Klaus about being a father, considering his own troubled past, could be a central part of the story.
Maybe Harry discovers his Mikaelson heritage when strange things start to happen to him during his time at Hogwarts. His newfound powers could attract the attention of both the wizarding world and the supernatural world from Klaus' side, leading to a complex web of alliances and rivalries. He might have to learn how to balance his wizarding skills with his vampire - like abilities.
Perhaps there could be a situation where a dark magic spell in the Potter universe somehow links to the Original vampire's curse in Klaus's world. Harry and Klaus are forced to work together to break the spell/curse. Along the way, Klaus sees a bit of himself in Harry's determination and Harry realizes that Klaus isn't entirely a monster.
Klaus would be more powerful. He's an Original vampire with super strength, speed, and the ability to compel others. He's also a skilled fighter. Kai is powerful as a witch, but Klaus has centuries of experience in combat and survival. His vampiric abilities give him an edge in a direct confrontation.
One interesting Klaus Mikaelson mate fanfiction could be 'The Bond Beyond Time'. It delves into the idea of a mate from a different era finding their way to Klaus. Another might be 'Mate Against All Odds' which focuses on a mate who has to face numerous challenges in the vampire - werewolf - witch world of Klaus Mikaelson. And 'Klaus's Hidden Mate' where the mate is initially hidden from Klaus due to some dark magic.
One popular Klaus Mikaelson pregnant fanfiction might involve a unique plot where the pregnancy is a result of a magical spell gone wrong. This could lead to all sorts of chaos and drama within the supernatural world he inhabits. Maybe other characters are forced to protect the pregnant Klaus due to the implications it has on the balance of power.
One idea could be a story where the baby Mikaelson has unique powers that start to manifest at a very young age. Maybe they can communicate with spirits, and this leads to all sorts of adventures as the family tries to protect the baby from those who might want to use their powers.
A third plot might be Elena getting stuck in Klaus' past through some magical mishap. She has to navigate his old world, meet his family when they were still alive, and in the process, she and Klaus grow closer as they face the challenges of that time together.
Well, there are fanfictions that play with the idea of Bella's mind - reading ability (if it was more developed) and how it would interact with Klaus's complex and often secretive mind. It could lead to some intense mental battles and also a deep understanding between them. Some stories might also explore a romantic relationship where Klaus, who is used to power and control, has to learn to be vulnerable around Bella. And vice versa, Bella has to adapt to Klaus's more brutal and ancient ways of dealing with things. They could face challenges from both their own 'families' in the respective fictional worlds, like the Cullens being wary of Klaus's intentions towards Bella and Klaus's siblings having their own opinions about his newfound interest.