Well, they could go back to the time when the first prophecy was made. Percy and Artemis would then be in a race against time to make sure the prophecy is set in motion correctly. Maybe they find out that some dark force from that time is trying to change the prophecy to gain power in the future. Also, they could end up in the past during a great war between the gods and have to influence the outcome in a way that ensures the balance of power remains in the future as it is supposed to. Another plot could be that they travel back to when Artemis first formed her Hunters and Percy has to help her make the right choices in choosing the first members, which will have a ripple effect on the future.
One idea could be that Percy and Artemis are sent back to the time of the Titans' rule. They have to work together secretly to gather allies among the minor gods and demigods who are oppressed. Another could be they go back to the founding of Camp Half - Blood and have to make sure the important events occur as they should while also dealing with a threat that could change the future of the camp. A third plot idea might be that they are thrown back to a time when a powerful artifact was first created, and they need to prevent it from falling into the wrong hands to safeguard the future of the demigod world.
One idea could be that after Percy's death, Artemis feels a strange sense of loss and responsibility. She starts to investigate the circumstances around his death more closely, which leads her to discover a hidden conspiracy that was targeting demigods. As she delves deeper, she also finds that Percy had left behind some sort of clue or power that she needs to figure out how to use in order to stop the conspiracy.
One plot idea could be Percy Jackson getting lost in the realm of Chaos and being found by Artemis. They then have to work together to find a way back to the normal world while facing strange creatures unique to Chaos. Another could be a story where Artemis is sent on a mission by Chaos and Percy joins her, and during the journey they discover a long - lost prophecy that involves both of them. Maybe a plot where Percy has a vision from Chaos about a threat to Artemis and he has to protect her while also dealing with his own internal struggles.
Maybe Goku goes back to the time when Planet Vegeta was still intact. He could try to prevent Frieza from destroying it. This would change the entire Saiyan race's fate and might create a whole new timeline with a powerful Saiyan empire.
One plot idea could be that Ino goes back in time to the founding of Konoha. She could use her knowledge of the future to prevent some of the early tragedies that befell the village. Another idea is that she goes back to her own childhood and tries to change certain events that led to insecurities in her later life. For example, she might try to befriend Sakura earlier and avoid the rivalry that developed between them.
One idea could be that the vanguard goes back to a crucial moment in history and has to prevent a major disaster that would change the future in a negative way. For example, they could stop a powerful weapon from being created too early.
One idea could be that Merlin and Elizabeth go back in time to Camelot's golden age. They could use their modern knowledge to solve ancient problems. For example, they might introduce new medical techniques that save lives. Another plot could involve them trying to prevent a major disaster in Camelot's past that they know about from history books. They might also have to deal with the cultural shock of living in a different time period.
Pan could also go back in time to meet his younger self. He could give his younger self advice on how to become stronger and more skilled. However, this might have unforeseen consequences as changing his own past could disrupt the balance of his life in the present. He might find that some of the relationships he had in the present are now completely different because of the changes he made in the past.
One idea could be that the demigods are sent back to prevent a great disaster that was actually caused by one of their own kind in the past. They have to figure out who it was and stop it without changing too much of the future.
One idea could be that Bella travels back to a time when vampires first emerged. She could try to change the history of vampire - human relations, maybe prevent some of the early vampire - made disasters. Another idea is she goes back to a time in her own family's history and discovers secrets that shape her present. For example, she might find out that her family has some hidden connection to the supernatural world long before she became involved with Edward.
One idea could be that Sasuke goes back in time and tries to prevent the Uchiha massacre. He could use his knowledge of the future to stop Itachi from carrying out the order. Another could be that he goes back to his early days at the ninja academy and tries to change his relationship with Naruto from the start. Maybe he would be more open and friendly, which could change the whole dynamic of their rivalry.