Perhaps they get into an argument over a decision regarding the safety of the kingdom. Cassandra wants to take a more aggressive approach while Rapunzel wants a peaceful solution. They then have to find a way to reconcile their differences and work together again. This could involve them both learning from each other's perspectives.
Another plot could be that Rapunzel is kidnapped by a mysterious figure from her past that she doesn't remember. As she's held captive, she starts to have flashbacks of her early childhood and discovers secrets about her family. Meanwhile, Flynn Rider and the others are on a frantic search for her, and Rapunzel has to find a way to signal them while also dealing with the revelations about her past.
One idea could be them going on a new adventure outside of Corona. Maybe they discover a hidden land full of magical creatures that they have to protect together. Another could be Flynn teaching Rapunzel some self - defense skills in a more comical way. For example, he tries to show her how to use a sword but she ends up using it like a gardening tool at first.
One idea could be that Naruto is on a mission in a strange land and stumbles upon Rapunzel's tower. He helps her escape from Mother Gothel, and in return, Rapunzel uses her hair to help Naruto in his ninja battles. Another could be that they both get transported to a third, unknown world and have to rely on each other's unique skills to survive and find their way back home.
A male Rapunzel could also be part of a story where his long hair has magical properties. Instead of the traditional healing magic often associated with Rapunzel's hair in the original story, his hair might have the power to control the elements. For example, he can summon wind to keep unwanted visitors away from his tower or use water to create a protective barrier. A group of friends might come across his tower, and together they figure out how to use his hair's powers for the greater good and to finally free him from his captivity.
I really like 'Rapunzel and Cassandra: The Unseen Adventures'. This fanfiction takes a different approach. It imagines all the adventures that Rapunzel and Cassandra might have had that we didn't see in the original story. It's full of action and there are some really interesting character developments for both of them. For example, Cassandra learns to be more open about her feelings, and Rapunzel becomes more confident in her leadership skills.
What makes it good is the exploration of their dynamic. In a fanfiction, we can see more of their private conversations, their unspoken thoughts. For example, how Cassandra really feels about Rapunzel's popularity. The setting also matters. If it's set in a different time or a parallel universe, it can add a whole new layer of interest. Good fanfictions also stay true to the essence of the characters. They don't make Rapunzel completely out of character by being overly aggressive or Cassandra overly meek. It should feel like a natural extension of the original story.
One idea could be that Varian creates a special invention for Rapunzel. Maybe it's a device that can help her better manage the kingdom's resources. For example, it could analyze the soil in Corona to determine the best crops to plant each season. Another plot could involve them going on an adventure together outside of Corona. They might discover a new magical place that holds the key to solving a problem in their kingdom. Or perhaps Varian gets into some trouble with his experiments, and Rapunzel has to use her leadership skills and kindness to help him out and also convince the people of Corona not to be too hard on him.
Another plot could involve Rapunzel's hair having the ability to repel Pitch Black's nightmares. Pitch discovers this and tries to cut off her hair. However, Rapunzel uses her wits and the help of Pascal and Maximus to outsmart him. In the process, Rapunzel realizes she has more power within her than she ever knew, not just in her hair but in her spirit, and she becomes a force to be reckoned with against Pitch.
Perhaps Rapunzel and Flynn find a mysterious old book that tells the story of a long - lost power. They then embark on a journey to discover if this power is real and, if so, how to use it for the good of their kingdom. Along the way, they face challenges like dangerous traps and a rival who also wants the power for himself.
One plot idea could be that Eugene has to go on a dangerous quest alone to find a magical herb to cure Rapunzel when she falls ill with a strange disease that normal medicine can't heal. Another could be a story where Rapunzel and Eugene accidentally swap bodies due to a magical mishap and they have to figure out how to reverse it while also dealing with the daily tasks in each other's bodies.