One common theme is her relationship with other characters. For example, how she interacts with the heroes like Finn. Another theme could be about her power over water and how it is used in different situations.
The main characters are Finn, a brave and adventurous human boy, and the Water Princess. Finn is known for his heroics and his sword - fighting skills. The Water Princess has control over water and is from the Water Kingdom.
One idea could be that the Water Princess discovers a hidden underwater kingdom that has a connection to the Land of Ooo. She has to team up with Finn and Jake to protect it from an evil sea creature.
Another option is to check out Tumblr. There are often fanfiction writers who post their works there. You might find some really interesting 'Adventure Time Water Princess' fanfictions that are different from the mainstream ones. Some might even combine her story with other Adventure Time plotlines in a really cool and unexpected way.
Well, they could start by having a bit of a misunderstanding. Finn might accidentally break a water - related tradition in the Water Princess's kingdom. This causes some tension between them at first. But then, they discover a common enemy that is threatening both the Land of Ooo and the Water Kingdom. So they have to put aside their differences and team up. They could go on a journey through different elemental regions, like a fire region where they have to use the Water Princess's powers cleverly to survive.
The Ice King could be a great antagonist. He might be jealous of the Water Princess's power over water and try to freeze all the water in her domain. His craziness and his desire for power could drive him to do this.
One idea could be that the Water Princess discovers a hidden underwater kingdom that was once part of her domain but has been lost. Another could be her facing a drought in the Land of Ooo and having to use her powers in new and creative ways to bring water back. Maybe she has to team up with Finn and Jake to fight an enemy that is polluting the water sources.
One possibility could be a story where Finn and the Lizard Princess go on a quest together. Maybe they have to find a magical item that is hidden deep in the Candy Kingdom. Finn's bravery and the Lizard Princess's unique abilities would surely make for an interesting adventure.
Well, in a fanfiction about Flame Princess' jealousy, it could be related to how others perceive her relationship with Finn. For example, if the other characters in Ooo start gossiping that Finn is too good for her or that their relationship won't last. This could make Flame Princess jealous of the opinions others have. She might try to prove them wrong by being overly affectionate with Finn in public, which could be awkward for Finn at times.