A cool idea for a fanfiction is that Elena and Caroline accidentally travel to a different dimension during time travel. In this dimension, the rules of vampirism are different, and they have to figure out how to get back while also dealing with new threats and allies. This would make for an exciting adventure - filled story.
One interesting fanfiction could be where Elena and Caroline travel back to the founding of their town. They might interact with the original founders and change the course of some of the town's dark secrets. This would add a whole new layer of drama and mystery to the story.
There could be a popular one where Elena and Caroline time travel to the era of the Originals when they were first turned. They get involved in the power struggles of that time and have to use their wits to survive. This would be full of political intrigue and action, which many fans might find appealing.
One idea could be that Elena and Elijah travel back to a time when their families had a different relationship. They could try to change the course of history to prevent some of the tragedies that happened to their families. For example, they might stop a feud that led to a lot of bloodshed.
There are also fanfictions where Klaus kidnaps Caroline as part of a deal or a bet. But as time passes, he finds himself intrigued by her strength and spirit. Caroline, on the other hand, uses this situation to try and figure out Klaus's weaknesses, which makes for an interesting power play between them.
One interesting RWBY time - travel fanfiction could be 'Back in Time at Beacon'. It often has the characters going back to their days at Beacon Academy and changing certain events. Another is 'Time - Twisted RWBY' where the time travel is more accidental and they have to figure out how to get back while dealing with altered timelines. And there's also 'RWBY: Through Time's Gate' which focuses on a magical gate that causes the time travel and the characters' adventures in different time periods.
Some Zuko time travel fanfictions involve him traveling to the future. In this scenario, he gets to see the world that has been rebuilt after the war. He might learn new bending techniques or ways of living and then take that knowledge back to his own time to help in the reconstruction of the Fire Nation.
One interesting Kushina time travel fanfiction could be where she travels back to prevent certain tragedies in the Naruto world. For example, she might go back to stop the Nine - Tails attack on Konoha. This would change the entire course of the story, saving many lives and altering the fates of characters like Naruto.
There might be a fanfiction where Sasuke travels to a time during the Fourth Great Ninja War but in an alternate timeline. He could interact with different versions of his friends and enemies. For example, he might meet a Naruto who has a different set of powers or a different personality because of the changes in the timeline. This would create a lot of interesting plot twists and character development.
One interesting Merthur time travel fanfiction could be 'Back to the Beginning'. It takes Merlin back in time to the day he first arrived in Camelot and he has to navigate his relationship with Arthur all over again, but this time with the knowledge he has from the future. Another great one might be 'Time's Twist' where Arthur travels to the future and sees how Merlin has been waiting for him for centuries. There's also 'Journey Through Time' which has both of them traveling through different eras of Camelot's history, changing things and facing the consequences.
A plotline could be that Elena and Caroline go back in time to a period before they knew each other. They then get involved in each other's lives in different ways than in their original timeline. This could lead to different relationships forming, not just between them but also with other characters like Stefan and Damon. They might also go back to a time when a major threat to the supernatural world was just emerging and they try to stop it before it becomes a big problem in the future.
Perhaps in the fanfiction, Stefan marries Elena and Caroline in a parallel universe where the rules of love and relationships are different. Stefan realizes that he has deep feelings for both of them and decides to marry them in a grand ceremony. This leads to a series of adventures as they try to merge their different worlds and deal with the jealousy and insecurities that come up among the three of them.