One idea could be exploring how the future technology in 2015 from the fanfic affects the daily lives of the characters more deeply. For example, how do the flying cars impact traffic rules and social interactions? Another could be a new character introduced in the fanfic who has a special ability related to the time - travel technology and how that changes the story's dynamic. Maybe this character can manipulate time in a different way than the original characters, leading to new adventures and challenges.
Well, perhaps a plot idea could be that in the 'Back to the Future 2015 Fanfic', the alternate 2015 has a different version of Marty's family. His parents could be in different careers, and Marty has to deal with the consequences of this new family structure. It could also involve the return of an old villain from the original movies but with a new, more devious plan in the fanfic's 2015 setting. This would create a lot of conflict and new storylines for Marty and Doc to navigate.
One idea could be a crossover with 'Doctor Who'. The DeLorean could time - travel to a universe where the Doctor exists, and they team up to fix a time rift that's threatening both their timelines. Maybe Marty and the Doctor have to figure out how to combine the technology of the TARDIS and the DeLorean to set things right.
Maybe in the RWBY Future Fanfic, the characters could explore new kingdoms. They might discover hidden secrets about the origin of Dust and how it can be harnessed in more powerful ways. This could lead to new power-ups for the characters and also introduce new allies and enemies.
A plot idea could be that the Time - Turner in Harry Potter is actually an ancient magical version of the Flux Capacitor. Hermione discovers this connection while researching in the library. She then tries to build a device that can combine the two technologies. This leads to her accidentally traveling to the Back to the Future universe. There, she meets Marty McFly and they have to work together to fix the time rift that her experiment created.
Another plot could be that in the future, the boundaries between the real world and Lyoko start to blur. The Lyoko Warriors have to find a way to stop this from causing chaos. They might encounter old enemies in new forms and have to use their past experiences combined with new knowledge to save both worlds.
One idea could be that Ciel has finally achieved his revenge, but now he finds himself lost without a purpose. He then discovers a new threat to the underworld that he feels compelled to fight against. This could involve new characters and a whole new set of mysteries.
One idea could be a crossover with the Dragon Ball Super universe where Future Trunks teams up with Goku Black from an alternate timeline. They could be forced to work together against a new, powerful enemy that threatens both their timelines. Another plot could involve a crossover with the Naruto universe. Future Trunks could end up in the Hidden Leaf Village and learn about chakra. He might then combine his ki powers with some ninja techniques to fight a common foe. A third idea is a crossover with the One Piece world. Future Trunks could land on the Grand Line and join forces with the Straw Hat Pirates. They would have to face off against the Marines and other pirates while also trying to find a way for Trunks to get back to his own world.
One idea could be Naruto going back in time and preventing the Uchiha massacre. He could use his knowledge of future events to change things for the better in the Uchiha clan, and this would also have a huge impact on Sasuke's character development.
In a 'Percy back in time fanfic', Percy could travel back to the time of the Titans' rule. He would be in a world full of chaos and oppression. Maybe he teams up with some of the early heroes who were fighting against the Titans secretly. He could learn new powers or fighting techniques from that era. His presence there could also cause some of the Titans to be more cautious, which could potentially change the outcome of the Titan - god war. This would then have a huge impact on the present that he came from, as the world would be very different in terms of power dynamics and relationships between different mythological beings.
One idea could be a crossover with 'Doctor Who'. The TARDIS could land in Hill Valley, and the Doctor could team up with Marty McFly. They could use the time - traveling capabilities of both the DeLorean and the TARDIS to fix some major time - related disasters across different universes.
Well, you could write a 'Back to the Future' fan fiction about a time when the DeLorean time machine malfunctions and sends Marty and Doc to a completely different era, like ancient Rome. They'd have to figure out how to get back while dealing with the culture shock and potential historical impacts they might make. Or perhaps a story where a future version of Marty comes back to his present time to warn himself about a coming disaster related to time travel. Another option is to create a fan fic where the time machine's flux capacitor starts to act up and creates mini - time portals all over Hill Valley, causing chaos that Marty and Doc have to sort out.