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.
In a 'Back to the Future' fan fiction, you could explore what would happen if there were other time - traveling devices in Hill Valley. Maybe a rival inventor creates a different kind of time machine and tries to outdo Doc Brown. This could lead to a race through time to achieve some important goal. Also, a story could be set where Marty has to go back in time to save Doc Brown from a danger that Doc doesn't even know about yet. It could involve a lot of detective work in the past to figure out the threat. And finally, you could write about how the events of the movies affected the lives of the minor characters in Hill Valley in ways that weren't shown in the original films, like the local mechanic or the school principal.
One interesting concept could be exploring different time periods that the characters didn't visit in the original movies. For example, what if Marty went back to the Middle Ages? Another could be alternate endings, like if Doc Brown had never invented the time machine. And some fan fictions might focus on the relationships between the characters in new and unexpected ways.
One idea could be exploring the character's backstories more deeply. For example, Duncan's life before the summer at the water park. What made him so shy and reserved? Another could be creating an alternate ending where Duncan stands up to Trent in a more dramatic way and changes the whole dynamic at the park. It could also be interesting to write a fan fiction that focuses on the relationship between Susanna and Owen, and how it developed over time.
One idea could be exploring a backstory for one of the minor characters in the series. Maybe a character who was only briefly shown but had a mysterious past.
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.
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.
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.
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.
Here's a more detailed one. Blaine has been traveling the world trying to forget Seb but can't. Seb has stayed in their hometown building a music career. One day, Blaine returns for a family reunion. He goes to a local music club where Seb is performing. Their eyes meet during the performance. After the show, they start talking and find out that neither of them has truly moved on. They then start spending time together, going to old haunts, and slowly they start to fall in love all over again. As they do, they have to deal with the memories of what went wrong before and how to make it better this time.
Well, you could write a fan fic where the relationships between the characters are reversed. So if in the original there are certain rivalries, in the fan fic they become allies and vice versa. Also, a story about a character who loses their power but discovers a new way to be strong within the 'unordinary' world would be interesting. Or how about a fan fic that takes place in a parallel dimension within the 'unordinary' universe where the rules of powers are slightly different?
Some interesting ideas for Riverdale fan fiction could be a crossover with another popular teen drama. Imagine the characters of Riverdale meeting the characters from 'Stranger Things'. It would be really cool to see how they interact. You could also write a fan fiction that focuses on the future of Riverdale. What will happen to the town and its residents ten years from now? Will Archie still be in town? How will Veronica's business empire develop?