Well, Natsu might time travel to a time when the Fairy Tail guild was just being founded. He could help the founders in ways they never expected, like bringing in some modern - day ideas for guild rules and relationships. He could also travel to a time when a powerful magical artifact was first created and make sure it doesn't fall into the wrong hands. Additionally, he could time travel to a period when a certain magic was lost and be the one to rediscover it, thus changing the course of magical history.
One plot idea could be Natsu traveling back in time to prevent a great disaster that he has knowledge of in the present. For example, he could go back to stop the rise of a powerful dark wizard before they gain too much strength. Another could be Natsu traveling to the past to meet his long - lost family members and learning about his past and heritage. He might also time travel to the future to see how his friends and the guild have changed and use that knowledge to better the present.
Well, Natsu could time travel to a time when the different dragon slayers were all being trained by their respective dragons. He could interact with them, learn new dragon slayer techniques, and form alliances that could help in his own time. Another idea is that he travels to a time when there was a great imbalance in magic power in the world, and he has to find a way to restore the balance. He could also go back in time to meet a long - lost relative or an important figure in the history of magic and get some crucial information or power from them.
One idea could be that Natsu goes back in time and accidentally changes something small that has a huge impact on the future. For example, he might prevent a key encounter that led to the formation of his guild, Fairy Tail, and then has to find a way to fix it.
A plot could involve Natsu going back in time to a time of great danger for the guild, a threat that was previously unknown in the present. He has to use his present - day skills and knowledge to fight this threat. But he also discovers some secrets about the guild's founders and their powers that were lost over time. This knowledge could be crucial in not only saving the past but also making the present - day Fairy Tail even stronger.
One idea could be that Natsu discovers a new and rare type of fairy magic that he has to learn to control. Another could be Natsu getting involved in a fairy war and having to choose a side. Maybe he could also meet a fairy who has the power to grant wishes and he has to decide what to wish for.
One interesting plot could be Natsu going back in time to prevent a great disaster that happened in the past of his world. For example, he might try to stop the initial attack of a powerful dark guild that had long - lasting negative impacts. Another plot could involve Natsu traveling back to his own childhood and having to deal with his past self, which could lead to some comical and heartwarming moments as he tries to impart some of his current wisdom.
One idea could be Yoda traveling back in time to train Anakin Skywalker differently, thus preventing his fall to the dark side. Maybe he goes back to the very beginning of the Jedi Order to gain more knowledge about the Force and bring it back to his own time.
One idea could be Shinji traveling back in time to prevent the Second Impact. He might try to change the actions of his father that led to it. Another could be Asuka traveling to a future where the Evangelions have been completely forgotten and she has to figure out how to get back to her own time.
Another plot could be a character from the future coming back to the present in Shameless. They could be trying to stop a future disaster that is somehow linked to the Gallagher family. This would involve a lot of mystery and intrigue as they try to figure out what the future event is and how to prevent it while dealing with the chaos of the present - day Gallagher family.
One idea could be Goten traveling back to the Saiyan - Saiyan war era. He could end up changing some small events that have big impacts on the future, like uniting some Saiyan clans earlier. This would create a whole new timeline where relationships between characters are different.
Maybe a character travels forward in time and sees a dystopian version of the ACOTAR world. They then have to go back and try to prevent the events that led to that future. They could face many challenges along the way, like not being believed or having to work against powerful forces who benefit from the future they saw. It would be a race against time to save their world.