An OC who has the ability to see the past and future of the world related to the magic of the Guardians. This OC starts having visions of a great disaster that will befall the Guardians and the world if they don't change their ways. So, the OC tries to convince each Guardian to make changes, which leads to a lot of self - discovery for the Guardians and a new bond with the OC.
One possible plot could be that your original character (OC) is a new spirit of creativity. The Guardians initially don't trust this new spirit as they are cautious of any disruptions to the balance they maintain. But as time goes on, they realize that the OC's unique power of creativity is exactly what is needed to combat a new threat that is draining the world of its color and imagination. Through a series of adventures, the OC earns the respect and friendship of the Guardians.
In this fanfiction scenario, Jamie could be kidnapped by a group of nightmare creatures who have broken away from Pitch's control. They might believe that Jamie holds the key to a new world order where nightmares rule. The Guardians would have to work together more closely than ever. North would use his workshop and his knowledge of the world to strategize. Bunny could use his tunnels to get to places quickly, and Sandy would send out his dream sand to try and communicate with Jamie in his sleep, all while trying to figure out how to defeat these rogue nightmare creatures and rescue Jamie.
One possible plot could be that Jack Frost is corrupted by a dark force. He starts to spread winter not as a fun, playful season but as a harsh, unforgiving one. He might turn against the other Guardians, believing they are holding him back from his true power.
A plot might involve a magical artifact that links the two universes of The Loud House and Rise of the Guardians. The Loud kids and the Guardians both become aware of it. They must work together to protect it from falling into the wrong hands. The older Loud sisters could use their creativity and resourcefulness, like Lori's ability to organize and Leni's fashion - inspired creativity, to help the Guardians in their quest.
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.
Another plotline could be the OC being in danger, and Jack Frost having to rescue them. This could be because the OC has a special power that some evil force wants to steal or because the OC has accidentally stumbled into a dangerous situation while exploring the magical world. Jack Frost then uses his ice powers and his wits to save the OC and keep them safe.
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.
The plot mainly focuses on a group of Guardians. They are characters like Santa Claus, the Tooth Fairy, etc. They come together to protect the hopes, beliefs and dreams of children from the threat of Pitch Black. It's a story full of magic and adventure.
The main plot involves a group of guardians like Santa Claus, the Easter Bunny, etc. coming together to protect the hopes, beliefs and dreams of children from the threat of Pitch Black, who wants to spread fear and darkness. They have to use their unique powers and work as a team to face this danger.
To create a believable OC, start with a concept that fits within the Rise of the Guardians universe. Let's say your OC is a spirit of hope. Consider how this concept of hope can be manifested as a power. It could be the ability to make people see the best in themselves. Then, develop their character through experiences. If they have witnessed a lot of despair, it could shape their view of the world and their role in it. Their relationships with the established characters should be developed organically. For example, they might initially be distrusted by Jack Frost because he's used to being alone, but over time they form a strong bond.