You could write a fanfiction where Tooth Fairy has to go on a solo mission to a far - off land to retrieve a special tooth that has the power to restore the health of all the fairies in her realm. Along the way, she discovers new things about herself and her powers.
In a fanfiction, the characters from 'Young Justice' could accidentally stumble into the realm of the 'Rise of the Guardians'. There, they are mistaken for enemies at first. But as they start to understand each other's motives, they realize they need to work together. For example, Superboy could learn a thing or two about controlling his powers from Jack Frost, while Jack Frost could learn about loyalty and teamwork from the Young Justice crew. This would be a great way to blend the two universes and explore different character relationships.
One idea could be exploring Jack Frost's past before he became a Guardian. Maybe he had a family or a life that he doesn't remember. Another idea is to create a new villain that challenges the Guardians in a different way than Pitch. For example, a villain who can manipulate time and mess with the memories of the children the Guardians protect.
One interesting plot idea could be Hiccup befriending Jack Frost from Rise of the Guardians. Maybe they exchange their skills, like Hiccup teaching Jack about dragons and Jack showing Hiccup some cool ice tricks. Another could be that they team up to fight a common enemy that threatens both their worlds, like a dark force that can control ice and also enslave dragons.
One idea could be exploring Pitch Black's past before he became the villain. Maybe he had a family or a lost love that led him down his dark path.
Another interesting concept is a story where the Guardians have to protect Hogwarts from a new dark threat. The Guardians, with their unique powers, team up with the Hogwarts students and teachers to fight off the evil. For example, Jack Frost could use his ice powers to create barriers, while the wizards use their spells.
One idea could be exploring Pitch Black's past in more detail. Maybe he had a friendship before he turned dark that could be rekindled in the fanfiction. Another idea is to have Pitch Black team up with the Guardians against a new, even greater threat.
Perhaps a plot where Pitch is injured and Jack, out of a sense of fairness or maybe just curiosity, decides to help him. This could lead to a strange bond forming between them. As Jack nurses Pitch back to health, Pitch starts to see Jack in a new light, not just as an annoying Guardian. And Jack gets to see the more vulnerable side of Pitch. This could create a lot of internal conflict for both characters as they deal with their new - found relationship within the context of their opposing natures.
One idea could be that the OC has a special power that Pitch Black is initially intrigued by and tries to use for his own dark purposes. For example, if the OC can manipulate dreams in a way different from the Guardians. Another plot could involve the OC being caught in the middle of the eternal struggle between Pitch and the Guardians, and having to make a choice about which side to ultimately support. Or perhaps the OC has a past connection to Pitch that is slowly revealed throughout the story, changing their relationship dynamic.
One idea could be that Jack Frost from 'Rise of the Guardians' visits Arendelle during a particularly harsh winter. He meets Elsa and they both discover that they have a unique connection to winter. Maybe Jack can teach Elsa some new tricks about controlling snow and ice, and in return, Elsa shows Jack how she uses her emotions to create beautiful ice sculptures. They could team up to save Arendelle from an even greater threat, like a dark force that wants to plunge the kingdom into eternal winter.
Perhaps the 12 Guardians are sent on a mission to a far - off land in the Naruto universe. Along the way, they encounter different cultures and powers. They have to adapt and learn from these new experiences in order to complete their mission. This could also involve some of them forming unexpected alliances with local characters and having to make difficult decisions between their loyalty to the group and their newfound friends.