Well, a stormtrooper might travel back to the time when the Jedi were still prominent. This could lead to some really cool interactions. Maybe he's on a mission to change the future of the Empire by eliminating a key Jedi in the past. Or perhaps he accidentally alters the timeline in a way that creates a whole new set of problems for the galaxy.
One idea could be a stormtrooper traveling back in time to the Clone Wars era. He could find himself in the middle of a battle and have to decide which side to support, if any. Another plot could involve time traveling to a future where the Empire has fallen and seeing how the galaxy has changed.
One idea could be an original character traveling back to the time of Aegon's Conquest. They could try to change the course of history by preventing certain alliances from forming. For example, they might stop the Starks from allying with the Targaryens, which would have huge ramifications for the future of Westeros.
At first, the stormtrooper's character could be very one - dimensional, just following orders. However, time travel would expose him to different cultures, beliefs, and ways of life. He might meet rebels and see their cause as something more noble than he thought. This could lead to a slow transformation where he becomes more independent - minded and starts to make his own decisions rather than blindly following the Empire's commands. He could also develop new skills as he adapts to different time periods and situations, becoming a more well - rounded character.
One idea could be that the OC is a new traveler in the Land of Ooo. They accidentally save the lives of several main characters' love interests. As a result, these characters become attracted to the OC and form a sort of harem around them. For example, Princess Bubblegum's scientific assistant could be among them, drawn to the OC's unique knowledge from another land. Another plot could involve an OC who has a special power that is needed to save the harem members from an ancient curse. This would create a lot of bonding and relationship - building moments.
One idea could be an OC who has a special ability to communicate with a previously unknown species in the Land Before Time world. This could lead to new alliances and adventures for Littlefoot and his friends.
One idea could be an OC that is a new type of dinosaur species that has unique abilities, like being able to communicate with certain plants. This OC could then help the main characters in their adventures, perhaps by finding a hidden path through a dangerous area.
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.