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.
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.
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 across time is also a prevalent theme. Stefan might meet someone in a different time period and fall in love. This creates a complex situation as he has to deal with the differences in their time periods and the fact that their relationship may not be sustainable due to the nature of time travel. There are often emotional and difficult decisions involved in these love stories within the time - travel fanfiction.
Another theme is the development of characters' emotions over time. As Stefan navigates his love for the two, we see how Elena and Caroline also grow and change in their relationships with him and with each other. For example, in some fanfictions, they might start off as rivals but end up as friends who understand each other's connection to Stefan.
One common theme is love and sacrifice. Their relationship often involves one or both of them having to sacrifice something for the other. For example, Stefan might sacrifice his own safety to protect 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.
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.
Love and relationships are also common themes. Time travel can bring new love interests into Caroline's life or test her existing relationships. Maybe she meets an old acquaintance in a new time period and their relationship takes a different turn.
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.
Another plot could be that they travel to a different time period and get stuck there. Then they have to figure out how to get back while also dealing with the challenges of that era, like different social norms and magic rules. They may also form unexpected alliances or face new enemies in that time period. This makes for an interesting story as they adapt and try to find their way home.