A really cool plot twist might be that Xander, as a girl, travels back in time and falls in love with a character who is an ancestor of one of her friends in the present. This would create a moral and emotional dilemma for her. Also, she could find out that in this different time period, there is a prophecy about a female figure (her) that is related to saving the world from a great evil. So, instead of being a sidekick - like character as she was as a male, she becomes the central figure in the time - travel adventure, which is a huge change from the original Btvs storyline.
One way the story could be developed is that as a girl, Xander might face different challenges in the time - travel adventure. For example, in the Buffyverse, gender often plays a role in how characters are perceived and treated. She could use her new perspective to change events in the past or future. Maybe she'd have a different relationship with the other characters, like Willow and Buffy. She could be more empathetic or perhaps face more discrimination depending on the time period she travels to.
A great plot twist could be that when Danny travels back in time to stop Vlad from doing something bad, he actually ends up causing it. For example, by trying to prevent Vlad from getting his ghost powers in a certain way, Danny accidentally sets off a chain of events that leads to Vlad getting even more powerful and evil powers.
In some fanfics, a character travels back in time to stop a dark wizard from getting a powerful artifact, only to find out they were the one who accidentally led the dark wizard to it in the first place. It's a great plot twist that adds a lot of depth to the story.
In some fanfics, a time - traveler might end up being the cause of Grindelwald's rise instead of stopping it. For example, in 'The Unintended Consequence', the main character's actions in the past accidentally give Grindelwald more power.
One interesting plot twist could be Naruto changing the past in a way that completely alters the future relationships. For example, if he manages to prevent certain tragedies from happening to his friends' families, it could lead to very different dynamics. Maybe Sasuke's family wouldn't be massacred, and Sasuke would grow up in a completely different environment, which might change his entire attitude towards the village and Naruto himself. This could lead to a whole new set of adventures and challenges for Naruto as he tries to adapt to this new timeline.
One interesting plot twist could be that as a girl, Dan enters a male - dominated field. For example, in a sports story, she could be the first girl on an all - boy soccer team. This would lead to a lot of challenges and conflicts as the boys might be initially resistant to having a girl on the team. But over time, she would prove herself and change their minds.
They could time travel to a future where everything they knew is in ruins. In this desolate future, they find out that their actions in the past led to this destruction. Now they have to work together to go back in time again and correct their mistakes. Sakura uses her intelligence to figure out what went wrong, while Madara uses his combat skills to protect them during their journey through time.
A really cool plot twist would be if during their time travel, they find out that time itself is a sentient being or is being controlled by an unknown force. This force has been manipulating events in the Soul Society and the human world all along. Ichigo and Urahara then have to figure out how to deal with this new discovery, whether to fight it or try to negotiate, while also trying to find their way back to their own time.
Another cool plot twist is when they time travel and get stuck in the wrong bodies. James Sirius might end up in the body of someone else in the past, like a Death Eater, and has to figure out how to get back to his own body while also not ruining the timeline. It adds a whole new layer of complexity and humor to the story.
One of the most interesting plot twists could be when a character from the original 'The Godfather' story travels back in time and accidentally changes a major event that alters the whole family's future. For example, if Michael Corleone went back and prevented his father from being shot, it would completely change the power dynamics and the family's path in the mafia world.