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.
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.
You can find such fanfiction on FanFiction.net. It has a large collection of fan - created stories related to various shows and characters, including those centered around Stefan, Elena, and Caroline.
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.
There's also 'Stefan and Caroline's Adventure'. In this fanfiction, they go on a journey to find a cure for a new kind of vampire - related illness. It has a lot of action and at the same time, their feelings for each other are beautifully depicted. The author does a great job of creating a believable scenario for their relationship to grow.
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.
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.
You can try fanfiction websites like Archive of Our Own. It has a vast collection of fanfictions from various fandoms, and there's a good chance you'll find some great 'colin x stefan fanfiction' there.
There could be a story where they are in a dangerous situation together, and the intensity of the moment leads them to confess their love and make love as a celebration of life and their newfound connection. It would be full of action and passion.
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.