Elena might be a bit shocked at first but then would be very supportive. She would offer her own experiences and advice, even if they are from a human perspective. Bonnie could be worried about the magical implications of a vampire pregnancy and offer to use her magic to keep an eye on the baby's development.
Another plot could involve Caroline's werewolf friend Tyler coming back and being initially skeptical but then becoming a strong support for them during the pregnancy. This could add some drama as there are historical tensions between vampires and werewolves.
Stefan might become more protective. His usually brooding nature could turn into a fierce determination to keep his family safe. Caroline, on the other hand, would probably become more nurturing. She was always strong - willed, but now she would focus that on taking care of the baby. She could also become more of a mediator between Stefan and any potential threats to their family.
In such a fanfiction, perhaps the plot could involve a lot of drama. Stefan might initially be in shock at the unexpected pregnancies. There could be a rift between him and his friends due to the complex situation. He would have to figure out how to support both Elena and Caroline during their pregnancies, and also deal with the reactions of the Mystic Falls community. Maybe he would start to realize his deeper feelings for both of them as they go through this life - changing experience together.
It could start to develop in new ways. For instance, if they go back to a time when Stefan was still with Elena, Caroline might see a different side of him. This could either make her fall for him more or create some tension between them. But as they work through these issues, they could end up with a deeper and more complex relationship. They might also discover new things about each other that they didn't know before, like hidden fears or desires from their pasts, which would bring them closer together.
There is also 'The Unlikely Bond'. In this fanfiction, it starts with Caroline and Stefan being forced to work together on a school project. Their initial dislike for each other gradually turns into a deep connection as they discover more about themselves through the process.
Love and sacrifice are common themes. Often, one of them has to sacrifice something for the other, like Stefan might sacrifice his safety to protect Caroline. It shows the depth of their love.
Well, Stefan's relationship with Elena and Caroline would transform into something more family - oriented. They would have to rely on each other more than ever. There could be a lot of emotional moments as they navigate this new chapter. Elena and Caroline's relationship could evolve from just being friends to co - parents - in - a - way with Stefan. And Stefan would have to deal with the fact that his actions have not only affected his own relationships but also the dynamics within the whole group of friends in Mystic Falls.
I really liked the fanfiction 'Stefan and Caroline: Unlikely Allies'. In this one, they start off as just friends trying to solve a mystery in Mystic Falls. But as they work together, their feelings start to change. It's interesting because it focuses a lot on their personalities and how they complement each other. There are some really good action scenes too, which makes it more exciting.
Archive of Our Own (AO3) is a great place. It has a large collection of fanfictions, and you can easily search for 'Caroline x Stefan' ones. There are different categories and tags to help you narrow down your search.