A fanfic on Caroline being pregnant could start with the moment she discovers she's expecting. It could then explore her emotions - from shock and fear at first to gradually accepting and even looking forward to motherhood. The story could also involve her preparing for the baby, like decorating the nursery and buying baby clothes.
A great plot for a Caroline pregnant fanfic would be the mystery of the baby's father. If it's not clear who the father is at the start, Caroline could go on a journey to figure it out. Along the way, she uncovers secrets about her past relationships and discovers things about herself that she never knew before.
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.
Yes, there are likely some fanfics out there with such a plot. Fanfic writers often explore different 'what - if' scenarios in the Vampire Diaries universe. They might be interested in the dynamic between Damon and Caroline in a situation where they are expecting a child, which could lead to a lot of interesting character development and new storylines.
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.
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.
Klaus might be conflicted. He might see Caroline's pregnancy as a chance to have a family in a different way than he expected, or he could be worried about the implications for his power and status. Elena would probably be very protective, as Caroline is her friend. She would offer support and advice, especially since she has some experience with the supernatural world.
In this fanfic, it could be that Caroline's pregnancy is part of a prophecy. This would lead to a lot of speculation and action within the story. Different factions might try to either protect or manipulate her depending on which side of the prophecy they believe in. Also, her pregnancy could be a catalyst for a change in the power dynamics in the supernatural world. For example, if she has a child with a powerful being, it could shift the balance of power, and everyone would be vying for influence over the unborn child.
You can try fanfic websites like Archive of Our Own (AO3). It has a vast collection of fanfictions, and you can search for 'Vampire Diaries Damon and Caroline pregnant' in the search bar. There's a good chance you'll find some relevant fanfics there.