In such a fanfic, there could be a lot of drama. Maybe Klaus would become extremely protective of Caroline. He might try to keep her safe from all the threats in their supernatural world, like his enemies who could use her pregnancy against him.
In the fanfic, the pregnancy could be the catalyst for them to truly open up to each other. They might share their deepest fears and desires. Klaus could reveal his insecurities about being a father, fearing he'll be like his own father. Caroline could comfort him and they could bond over their hopes for the baby. As the pregnancy progresses, their love could grow stronger, and they could become a more united front against any threats.
Her friends might be shocked at first. Bonnie, for example, would probably be worried about the implications of having a baby with Klaus, considering his past. But she would also be there for Caroline, offering her magical support if needed, like protection spells for the baby.
Klaus would be very attentive to Caroline's pregnancy cravings. He'd see it as his duty to take care of her during this time. He might cook for her himself if he can, or take her to the best restaurants to get what she wants. He would be a bit overprotective too, making sure that whatever she eats is of the highest quality and safe for her and the baby. He'd also listen intently to her when she talks about her cravings, finding it endearing and a sign of the new life growing inside her.
Klaus might be overjoyed. He has always been attracted to Caroline, and having a baby with her could be the ultimate sign of their connection. He would probably be very protective, using all his power and resources to keep her and the unborn baby safe.
Their relationship could become more intimate as they bond over the baby. They might start to see each other in a new light, with Klaus showing a more tender side and Caroline learning to trust him more deeply.
It could become more intense. They now have a shared responsibility, so they might be more connected. Klaus might be more tender towards Caroline, and Caroline might start to see Klaus in a different light, perhaps more as a partner rather than just an acquaintance.
One idea could be that Klaus and Caroline initially struggle with the news. Klaus is worried about his enemies using Caroline and the baby as a target. Caroline, on the other hand, is determined to protect her unborn child at all costs. They might go into hiding together, which would lead to some sweet and tender moments between them as they get to know each other on a deeper level away from the chaos of their normal lives.