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.
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.
One idea could be that humans discover a secret alien base on Earth. They gather all their scientific knowledge and military might to launch a surprise attack on the aliens. Using advanced technology reverse - engineered from captured alien devices, they manage to fight back effectively.
A great plot for Strike the Blood fanfiction could be a time - travel scenario. Kojou is sent back in time to a period when vampires were first emerging. He has to use his modern - day knowledge and powers to survive in this ancient and dangerous time, while also trying to find a way back to his own time. This would allow for a lot of world - building and exploration of the vampire lore in the series.
Another plot could focus on the origin stories of some of the side characters. For instance, the backstory of Yukina Himeragi. How did she become an attack mage? What kind of training did she go through? A fanfic could expand on this and also show how her past experiences influence her actions and relationships in the present story.
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.
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 an OC who is a powerful vampire hunter sent to the island where the story takes place. They initially clash with the main characters but end up teaming up to face a greater threat. For example, a new type of ancient vampire that is immune to the usual methods of slaying.
Stonebridge is also very popular. His relationship with Scott is full of potential for fan fiction. You could write about their first meeting from Stonebridge's perspective and how he felt about joining the team. Additionally, exploring his personal life outside of the missions, like his family or hobbies, could add depth to his character in fan fiction.
One idea could be a story where the Mandalorian has to team up with an old enemy to fight a new, greater threat. Maybe a powerful Sith Lord emerges and they have to put aside their differences.
One idea could be exploring a different origin for Toothless. Maybe he was part of an ancient dragon tribe that was thought to be extinct. Another could be a story where Hiccup discovers a new dragon species that has unique abilities and forms an alliance with them. A third idea is a time - travel fan fiction where Hiccup goes back in time to prevent a great dragon war from ever starting.