A possible plot is that Bellatrix is sent back to the time when the Goblin Rebellions were happening. She could side with the goblins to gain their trust and then double - cross them to steal their most precious treasures. Another idea is that she is sent back to the time when the Ministry of Magic was first established. Bellatrix might try to infiltrate it from the start and plant the seeds of dark magic within its very foundation. Also, she could be sent back to a time when a famous magical beast was first discovered, and she tries to capture and control it for her own evil purposes.
A really interesting plot could be Bellatrix being given a chance at redemption. In this fanfiction, she could start to question Voldemort's motives after seeing the destruction he causes. She might then try to secretly help the Order of the Phoenix. Also, a plot about her teaching a young Death Eater the ways of the Dark Arts and having second thoughts about it mid - way through could be fascinating.
Some fanfics explore Bellatrix's family relationships more deeply. Like her relationship with Narcissa. There could be a plot where Bellatrix has to choose between loyalty to Voldemort and protecting her sister, which adds a lot of emotional depth.
A possible plot is that Anakin is sent back in time and ends up meeting his mother again when she was younger. This encounter could give him a chance to save her from the Tusken Raiders, which might then have a profound impact on his emotional state and his path in the Force. He could also use this opportunity to train himself differently, perhaps learning from the Jedi of that era in a more positive way, without the strictures that later led to his disillusionment.
One idea could be exploring how Bellatrix's pregnancy affects her loyalty to Voldemort. Maybe she starts to question some of his more extreme actions as she thinks about the future for her unborn child.
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 Percy goes back to the time when the gods first came to power. He could try to change the way the gods interact with demigods from the start, leading to a whole new world order for demigods. Maybe he'd teach the early demigods some of the skills he has, like advanced sword fighting techniques.
Bellatrix's personality could be softened in some ways. Going back in time, she might see things differently. For example, if she goes back to a time when the wizarding world was more peaceful, she could start to question her loyalty to Voldemort. However, she could also become more desperate. If she realizes that she has a chance to change things for the worse in the wizarding world, she might be more eager to carry out her dark plans, and her already - extreme personality could become even more so.
Jon Snow could be sent back to a time just before Ned Stark's father was killed. With his future - learned wisdom, he could try to save his grandfather and thus change the entire Stark family's fate. He may also discover secrets about his own lineage that were lost in time. This could lead to him having more power and influence in the past, as he might be able to claim certain rights or titles based on this new - found knowledge.
One interesting plot idea could be Percy going back in time to the era of the Greek heroes. He could meet Hercules and learn from him. Maybe he would have to face the same challenges Hercules did, but with his own modern - day skills and knowledge. Another idea is that he goes back to the time when the gods first emerged and gets involved in the power struggles among them. This could change the entire course of mythology as we know it.
A possible plot could be that they are under the influence of a powerful mind - controlling entity. In this fanfiction, the entity forces them into a relationship and pregnancy. The story could then revolve around how they break free from the control and deal with the consequences of the pregnancy. It could explore themes of free will, redemption, and the power of love against dark forces.