A possible plot could be that Jaune's children time travel to a period where the balance of magic was disrupted. They then have to work with their father (the younger version) to restore the balance. They could also time travel to a time when Jaune made a big mistake in his life. His children, with their knowledge from the future, could help him avoid that mistake and set him on a better path. Another plot could be that they time travel to a moment in history where a powerful artifact was created, and they need to ensure it is created correctly so it can be used in the future to save their world.
Well, they might travel back to a time when Jaune was just starting out at a school. His children could be there to secretly guide him through difficult challenges, like passing a really tough exam or getting over a fear of public speaking. They could also travel back to a time of an ancient prophecy and make sure that Jaune's actions are in line with what needs to be done to fulfill it for the good of the future. And of course, they could go back to a time when a powerful enemy first emerged and try to stop it from growing in power.
Another idea is that they travel to a time before they ever met. This allows them to see how things were without their influence. Maybe they find out that small changes in the past have a huge impact on the future. For example, if Pyrrha hadn't been so well - known before coming to Beacon, how would that change the relationships and events there? And Jaune could see how different his life would have been without Pyrrha's initial guidance and friendship.
Maybe they travel to a time long before the events of the story, during a period when the kingdoms were first being established. They get involved in the ancient politics and power struggles, and in the process, they discover secrets about the origin of the Grimm and the power of the Relics that were long forgotten. Their actions in this past time could have a huge impact on their present world.
One idea could be Jaune's journey in training the Maidens and how he helps them unlock their full potential. Maybe he discovers a new form of combat that combines their Maiden powers with his own skills.
One idea could be Madara traveling back to the era of the Sage of Six Paths. He could try to change the very foundation of ninjutsu, which would create a whole new timeline. Maybe he would attempt to gain the power of the Sage for himself much earlier in history.
Well, perhaps a time - traveling device accidentally sends Judy Hopps back to a time when prey and predator relations were extremely tense. She has to use her wits and modern - day knowledge to prevent a major conflict that could have changed Zootopia's future for the worse. Or, a time - traveling fanfiction could focus on Nick Wilde going back to his own past, where he can make different choices that would impact his relationship with Judy and his standing in Zootopia society in the present.
One idea could be a character from the present in BO2 traveling back to a crucial moment in the game's history to change the outcome of a major battle. This would create a whole new timeline and a lot of interesting interactions with past characters.
Perhaps they travel forward in time instead. They could see the consequences of their actions in the future. Maybe they find out that their actions had unintended negative effects on some of their friends or the city as a whole. This would lead them to go back and try to fix things in a different way.
One idea could be Sauron travels back to the time before the forging of the Rings of Power. He could try to prevent the elves from ever having the knowledge to create such powerful items, thus changing the course of Middle - earth's history in his favor. Maybe he would target Celebrimbor directly, the elf - smith who was crucial in the creation of the rings.
In some narusasu time travel fanfiction, Naruto might go back to the time when Orochimaru was first targeting Sasuke. He could then use his skills and knowledge to keep Sasuke safe and away from Orochimaru's influence. A different plot could be Sasuke traveling to a time when Naruto was ostracized as a child and being the one to support Naruto, which would change their relationship dynamic from the start. Also, they could travel to a time during a major battle and use their combined future knowledge to turn the tides in their favor.
Another plot could involve Merlin traveling to the future. He might see a desolate Camelot, destroyed by some unknown force. Then he has to return to his own time and figure out how to prevent this dark future from happening. This would involve him using his magic in new and creative ways to influence the present to change the future.