Another plotline could be that they accidentally travel to the future and have to deal with the consequences of seeing their future selves. They may discover things about their relationship or the vampire world that they didn't expect. Sometimes, they are on a mission in the future to prevent a disaster from happening in their present time. This involves a lot of action and adventure as they try to figure out how to stop the impending doom while also dealing with the strange future world they find themselves in.
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.
You can try fanfiction websites like Archive of Our Own (AO3). There are a large number of fanfics about Stefan and Caroline time travel there. Just search for 'Stefan and Caroline time travel' in the search bar, and you'll get a bunch of results.
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.
They might change some crucial events in their past. For example, Stefan could prevent some of the bad things he did while being a ripper. Caroline, on the other hand, could go back to when she was a human and make different choices about her relationships or her path in life.
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.
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.
The theme of 'power couple' is also quite common. Fans like to write about how Caroline and Stefan, with their combined strength, intelligence, and vampire abilities, can take on the various threats in Mystic Falls. They become a force to be reckoned with, and these stories usually have them saving the day in spectacular ways.