The antagonist could be an entity that controls the flow of time. It sees Clay and Hannah's time - traveling as a disruption to the natural order. It tries to force them back to their original time, but in a way that could be harmful to them. This entity might not have emotions in the human sense but is just following some strict rules about time, and Clay and Hannah have to find a way to convince it or outsmart it to continue their time - traveling adventure.
It could be a time - traveling bully from another era. This person might be jealous of Clay and Hannah's relationship and try to keep them apart in different time periods. They could use their knowledge of different times to set up obstacles for Clay and Hannah, like making it seem as if they belong in different timelines and should not be together.
Maybe the antagonist is a future version of someone from their present who has become corrupted by power. This character realizes that Clay and Hannah's time - traveling could disrupt their plans for the future. So, they try to stop them from traveling through time and interfere with any positive changes they might make. They might use advanced technology or knowledge from the future to create difficulties for Clay and Hannah.
One possible plot could be that Clay and Hannah find themselves traveling through different eras in their town's history. They might end up in the past, seeing how things used to be and how the events of the past are connected to their present situation. For example, they could witness the founding of their school and realize how the values or problems from that time still resonate today.
The Riddler could be a great main antagonist. He could use his knowledge of time paradoxes and puzzles to try and outwit Batman as he travels through time. He might set up elaborate traps across different time periods that Batman has to solve.
A time - traveling pirate who has his own agenda and wants to change the course of pirate history for his own gain could be the antagonist. This pirate might be jealous of Luffy's future achievements and try to stop him from ever becoming the Pirate King by changing things in the past.
The themes usually include love and loss. Their relationship was complex, and in fanfics, the love they had and the loss of Hannah are explored deeply. Hope is also a theme. Clay might be searching for hope in the midst of his despair. Additionally, self - discovery is common as Clay tries to figure out who he is in the wake of Hannah's death. It's like he's redefining himself.
It could be a future version of an existing villain who has also found a way to time - travel. For example, a more powerful and cunning Orochimaru from a future timeline where he has mastered even more dark jutsu. He might try to stop Naruto from changing the past in ways that would prevent his own rise to power.
It could be a dark wizard from the past who somehow senses that Harry and Teddy don't belong in that time and tries to use their presence for his own evil gains. Maybe he wants to steal their future knowledge to become even more powerful.
There's also the possibility that the main antagonist is an outside force from another dimension or a powerful entity that was awakened in the future. This entity might see the ninja world as a threat or simply want to conquer it for its own gain. Naruto would need to figure out how to defeat this unknown and powerful foe during his time in the future.
It could be a time - traveling rogue ninja from the future who has his own plans to change history for his own benefit. He might want to take over the ninja world in the past so that he can rule in the future without any opposition.
It could be Orochimaru. Even in the academy days, he was already plotting and scheming. If Naruto time - travels back, Orochimaru might see him as a threat to his early plans and target him. Orochimaru's thirst for power and knowledge makes him a great antagonist for this scenario.
One common theme is friendship. Clay and Hannah's relationship often starts as a friendship that has the potential to grow into something more. Another theme is support. Clay is usually depicted as being very supportive of Hannah, through her various challenges.