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.
There's also 'Klaus and Bella: A New World'. This story imagines a scenario where Bella, after the events of 'Twilight', somehow ends up in the 'The Vampire Diaries' world. Klaus becomes interested in her unique situation and they start to develop a relationship that is both challenging and full of unexpected twists as they navigate the differences in their respective worlds.
Love against all odds is also a big theme. Since they are from different fictional universes originally, their relationship is considered taboo or at least very difficult. Fanfictions often explore how their love can overcome these barriers. For example, in some stories, they have to battle powerful enemies who don't want them to be together. And in the process, their love becomes stronger. This is a very engaging aspect for readers as it shows the power of love in a very dramatic way.
There's also 'Mates Across Realms'. This fanfiction delves deep into how their different worlds, Bella from the Twilight world and Klaus from The Vampire Diaries world, come together. It has a great plot that builds their relationship gradually. The author does an amazing job of creating scenarios where they have to rely on each other and in the process, fall in love.
AO3 is probably the best bet. It has a very organized tagging system. When you search for Bella Swan and Klaus Mikaelson, you'll get a list of stories sorted by popularity, kudos (which is like a 'like' system), and bookmarks. This makes it easier to find well - received fanfiction. Additionally, Tumblr can be a source. There are often bloggers who share and recommend good fanfiction. You can search for relevant tags like #BellaSwanKlausMikaelsonFanfiction and you might come across some great reads that other fans have shared.
Social media platforms can also be useful. Look for groups on Facebook or Twitter that are related to these two fandoms. Fans might post links to their favorite fanfictions or discuss which ones they think are the best. You can also use Tumblr, as it has a large fanfiction - sharing community.
Well, perhaps a plot where Bella has to hide her pregnancy from the Volturi. Since Klaus is not part of the normal vampire world they are used to, they might see this as a threat. Also, there could be a plotline about how Klaus himself reacts to the news. Is he going to be protective or indifferent? And that would create a whole new set of tensions and story developments.
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 Bella discovers the world of the Mikaelsons after being turned into a vampire. She crosses paths with Elijah and they form an unlikely alliance to deal with a common threat in the supernatural world. They learn from each other's experiences, with Bella's knowledge of the Volturi and Elijah's centuries - old wisdom.
A really interesting storyline could be Bella trying to reconcile her old life in Forks with her new life as a Mikaelson. She might struggle with the differences in values and lifestyles. For example, the Mikaelsons are more into power plays and manipulation in the supernatural world, while Bella was used to the more straightforward (although still complex) relationships in 'Twilight'. This internal conflict could be a major part of the story, along with her trying to find her place within the Mikaelson family and the wider supernatural community.
One plot idea could be Bella's struggle to adapt to the Mikaelson family's vampire - werewolf drama while also dealing with her own newfound vampire - like powers as part of the family. Another could be her relationship with Klaus. Maybe she tries to temper his more violent tendencies and in the process, they form a unique bond that changes the family dynamics.