Maybe in the fanfiction, Aang goes back in time and meets his past self. He then has to guide his younger self without disrupting the timeline too much. This could lead to some really cool moments of self - discovery for Aang, as he sees himself from an outside perspective and realizes how much he has grown.
One interesting plot could be Severus going back to his school days and trying to prevent certain tragedies. For example, he might try to stop the rift between him and Lily. He could use his knowledge of the future to avoid making the mistakes that led to their falling out. Another plot could involve him changing the course of events related to Voldemort's rise. Maybe he tries to expose Voldemort's plans earlier or influence key characters to oppose him more effectively. He could also go back to a time before he got involved with the Death Eaters and choose a different path entirely, which would then have a huge impact on the wizarding world as we know it.
One interesting plot could be Izuku going back in time and using his future knowledge to prevent certain tragedies. For example, he might stop the initial injury that All Might received, which could change the whole course of the story. Another plot could be him going back to his own childhood and being able to train better and earlier, thus becoming an even stronger hero. He could also use his knowledge of future villains to stop them before they become a major threat.
One interesting plot could be Snape going back and trying to prevent the death of Lily. He might use his knowledge of the future to change certain events that led to her demise. Maybe he would try to expose Pettigrew earlier or find a way to make James Potter more cautious.
Well, in 'Bilbo goes back in time' fanfiction, a possible plot is that Bilbo goes back to the time before Smaug took over the Lonely Mountain. He could then try to warn the dwarves and help them better prepare for the dragon's arrival. This might involve him teaching them new battle strategies or helping them find a better hiding place for their treasures. Another interesting idea is that Bilbo travels back to the time of the great elven kingdoms' prime. He could experience the height of elven culture and maybe even learn some ancient elven magic that he could bring back to his own time.
One interesting plot could be Stiles going back in time and trying to prevent a major tragedy from happening in the past that has consequences in his present. For example, he might stop a villain from getting a powerful artifact that later causes chaos in his own time.
One interesting plot could be Hermione going back in time and preventing certain tragedies from the original story. For example, she might stop the death of a beloved character. Another could be her using her modern - day knowledge to change the course of wizarding history, like introducing new spells or concepts earlier.
In some fanfictions, Harry goes back in time and decides to train himself better. He could seek out different magical knowledge or combat skills that he didn't have the first time around, which then changes his future interactions and battles. For example, he might learn ancient spells that give him an edge against Voldemort.
Sokka would also be a common character. His strategic mind could be very useful in a different time period. He might be the one who figures out how to deal with the new situations they encounter while in the past. For instance, if they need to outwit an enemy in a historical battle, Sokka would be the one to come up with a plan.
One interesting plot could be Naruto Uchiha using his future knowledge to prevent the Uchiha massacre. He might try to change the minds of the village elders and stop the cycle of hatred before it spirals out of control.
One interesting plot could be that female Naruto goes back in time and uses her knowledge from the future to prevent the destruction of her village. She might try to stop certain villains from emerging or change the fates of her friends who died in the original timeline. For example, she could stop Orochimaru's early evil plans, which would have a huge impact on the story's development.