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 could be a fanfiction where Bella, after the events in her own series, somehow ends up in the world of Klaus Mikaelson. She might initially be scared, but her strong will makes her stand up to Klaus. And as time passes, they start to see each other in a different light. Maybe she helps him deal with his family issues, while he protects her from the new threats in his world.
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.
Klaus would probably be very intrigued by Bella. He's used to getting his way with most people, but Bella is not easily swayed. So he might start by trying to charm her, but she would be resistant. For example, he could offer her power or protection, but she would be more concerned about her own values and the people she cares about in Forks.
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 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.