Another interesting concept is if an enemy from the past is brought to the present in a time travel accident. Maybe Salem is accidentally brought forward in time. This would force the RWBY characters to face her with their current skills and knowledge, leading to intense battles and strategic planning as they figure out how to send her back or defeat her in this new time.
A time - travel RWBY fanfic could also explore the idea of alternate timelines. A character like Weiss could travel to different timelines where choices made in the past led to completely different Remnants. She could then bring back knowledge or items from these alternate timelines to help in her own world's fight against the Grimm and other threats.
Maybe a character from the future RWBY world travels back to the time when the Huntsman academies were first founded. They could introduce new fighting techniques or ideas that were developed in their time, which would have a huge impact on how the early Hunters and Huntresses are trained.
One idea could be Ruby traveling back in time to the moment when Beacon Academy was first founded. She could then use her knowledge of future events to prevent certain disasters from occurring, like the Grimm attacks that would later plague the school. Another plot could involve Ruby going back to a time when her mother was alive and getting to know her, which would add a lot of emotional depth to the story. Maybe she could also change some events in her mother's life that would have a ripple effect on the present she came from.
Maybe in the fanfic, a villain has the stop - time ability. This causes chaos as they start stealing important artifacts while everyone else is frozen. The RWBY team has to find a way to counter this power, perhaps by teaming up with a time - wizard who teaches them how to move in stopped time.
One idea could be that a modern Beyblader gets transported back in time to the origin of Beyblading. He could then learn the ancient techniques and bring them back to the present, revolutionizing the sport. Another idea is that a past Beyblading champion is brought to the future to see how the game has evolved. This could lead to some interesting battles and exchanges of strategies.
One idea could be a fanfic that explores the backstories of minor characters in RWBY more deeply. For example, you could focus on a character like Velvet and create a story about her training before coming to Beacon. Another could be a crossover within the RWBY universe itself, like a team-up between different schools' teams that we haven't seen much of in the official series.
One idea could be Batman traveling back to the time when Gotham was first founded. He could try to prevent the seeds of corruption from being sown. Another plot could be him traveling to a future Gotham that has been completely overrun by villains and he has to find a way to change that future by altering events in the past. Or perhaps he travels to different time periods to gather knowledge and technology to better fight crime in his own time.
One interesting RWBY time - travel fanfiction could be 'Back in Time at Beacon'. It often has the characters going back to their days at Beacon Academy and changing certain events. Another is 'Time - Twisted RWBY' where the time travel is more accidental and they have to figure out how to get back while dealing with altered timelines. And there's also 'RWBY: Through Time's Gate' which focuses on a magical gate that causes the time travel and the characters' adventures in different time periods.
One interesting idea could be a character going back in time to prevent the initial titan invasion. They might use their knowledge of future events to change strategies and save more lives. For example, if Eren knew from the start how to better fight the titans, he could have trained the soldiers more effectively.
A cool plot could be that a villain from the future uses time travel to go back and change the past in order to gain ultimate power in the Arrow universe. The heroes then have to figure out a way to reverse the changes and stop the villain. This could involve a lot of action scenes as they race against time to save their world from this altered timeline.
A cool plot for a Pokemon time travel fanfic might be that a Pokemon researcher travels back in time to study ancient Pokemon species that are now extinct. Along the way, they encounter problems like not being able to use modern technology to catch or study the Pokemon. They might have to rely on the knowledge of the people in that era. Also, a trainer could travel through time to meet their ancestors who were also Pokemon trainers and learn about their family's history in the Pokemon world.