You could write a fanfic where the guardian angel loses its powers for a while. This would create an interesting situation as it has to rely on the help of others in the Land of Ooo. It could also lead to character development as it discovers new things about itself and the world without its angelic abilities. Maybe it forms new friendships or realizes the true value of certain things.
A cool fanfic concept would be a time - travel adventure involving the guardian angel. It could go back in time to a crucial event in the Adventure Time history and try to change things for the better. But of course, there would be unforeseen consequences. This would allow for a lot of creativity in exploring different eras of the Adventure Time universe and how the guardian angel's actions ripple through time.
The Ice King could be really interesting. He's always up to something crazy, and his relationship with the guardian angel could be full of comical misunderstandings. Marceline the Vampire Queen is another character. Her dark past and her friendship with Finn and Jake can add depth to the fanfic when combined with the presence of the guardian angel. Plus, she has her own unique powers and personality that can create great storylines.
One idea could be that the guardian angel has lost its memory and has to figure out its purpose while protecting its human charge. Another could be a story where the guardian angel falls in love with the human it's supposed to protect, which creates a whole set of moral and emotional dilemmas. Or perhaps the guardian angel has to team up with a fallen angel to save the human from a greater evil.
One idea could be Finn and Jake starting a new school year at a magical school in Ooo. They could have adventures while trying to pass their classes like Magic 101 taught by a wizardly character. Another could be Princess Bubblegum enrolling in a science - focused school to learn more about advanced chemistry and creating new candies. Maybe Marceline could attend a music school in the Land of Ooo where she discovers new ways to play her ax - bass and meets other musical creatures.
One idea could be a fanfic where Finn and Jake discover a new, mysterious land full of strange creatures that have powers they've never seen before. Another could be a story about Princess Bubblegum's early days as a ruler, like how she built the Candy Kingdom from the ground up. Or perhaps a fanfic that focuses on Marceline's relationship with her father and how it affected her growth as a vampire.
In some 'Harry Potter Guardian Angel Fanfiction', one interesting idea could be a guardian angel that takes the form of a beloved character who has passed away, like Sirius Black. This angel could secretly guide Harry through difficult times, using Sirius's knowledge of the wizarding world and his love for Harry.
One interesting 'guardian angel twilight fanfic' could be about a guardian angel protecting Bella from a new, unseen danger in Forks. Maybe it's a supernatural force that the Cullens are unaware of, and the angel has to use its unique powers to keep her safe.
You could create a fanfic where Finn and Jake have to go on a mission to a completely new and unknown land in the Adventure Time world for your FRT fanfic. They might encounter strange new creatures and have to use their wits and powers in ways they haven't before. Maybe they find a land where the rules of magic are different, and they have to adapt quickly.
A great adventure time genderbend fanfic idea could involve Lumpy Space Princess as a male. He could be really vain in a different way, perhaps more focused on his muscles or some other masculine trait. And for Flame Princess, if she were a male, his relationship with Finn would be really different. Instead of the love - hate relationship they had, it could be more of a bromance, with them competing in feats of strength or something similar.
In the 'harry potter has a guardian angel fanfiction', it might be interesting to see how the angel's presence affects Harry's relationships. Does it make his friends more curious about him? Or does it create some sort of jealousy among them? Also, how does the angel interact with Dumbledore? Does it work with him secretly to ensure Harry's safety?
One idea could be a fanfic where a new character is a guardian of Naruto's hidden powers. This character secretly protects Naruto from those who might try to steal or misuse his power.