Perhaps a plot where a new villain emerges who has the power to control the dragons and uses them to attack both the world of Brave and Tangled. The heroes then must find a way to break the control and defeat the villain. They could start by seeking out an ancient artifact that has the power to counter the villain's magic. Along the way, they face many challenges and learn about each other's strengths and weaknesses.
A cool plot could be that the dragons are actually the key to a hidden realm that holds the secret to a great power. The characters from Brave and Tangled accidentally discover this connection and embark on a journey to the dragon's lair. Once there, they have to pass various tests set by the dragons themselves to prove they are worthy of the power. This could involve feats of courage, solving riddles, and showing kindness.
Merida from Brave could be a main character. Her archery skills and fearless nature would be great assets in any adventure with dragons.
One idea could be that Rapunzel's long hair has some sort of magical connection to the underwater world of the Little Mermaid. Maybe her hair can be used to communicate with the mermaids or even has the power to purify the polluted parts of the ocean.
One idea could be that Hermione somehow gets transported to the world of Tangled. She could use her magic to help Rapunzel understand her powers better. Another could be Harry Potter himself going on a quest in the Tangled world to find a magical artifact that is crucial for the wizarding world but has been lost there.
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.
For a fanfiction plot, consider Rapunzel trying to teach other people in the kingdom how to use their own 'inner magic'. She could start a school or something similar. Along the way, she has to overcome the doubts of the more traditional members of the court. Another idea is that she gets transported to a different land where no one has ever seen magic like hers before. There, she has to prove herself and her good intentions while also finding a way back home.
Perhaps a plot where dragons have to team up with humans to save their shared world from a great evil. This could explore the relationship between the two species, with dragons bringing their powerful magic and flight abilities, and humans contributing their intelligence and resourcefulness.
One interesting plot idea could be the discovery of a long - lost dragon egg in 'the fall of dragons fanfiction'. This egg might hold the key to saving the dragon race from extinction. Another idea could be a power struggle among the remaining dragons for the last remaining territory. And perhaps a human - dragon alliance forms to fight a common enemy that is threatening both their existences.
One idea could be a story where a dragon rider discovers a new, hidden ability of their dragon, like the ability to communicate with other magical creatures. This could lead to all sorts of adventures as they explore this newfound connection.
Another plot could involve the twin dragons being cursed. They start to lose their dragon forms gradually and have to embark on a journey to find the cure. Along the way, they meet various characters who either help or hinder them, and they also learn more about their own history and the origin of the curse.
One idea could be a dragon's journey to find a lost magical artifact. Another could be a story about a dragon rider who has to overcome a curse. Or perhaps a plot where dragons from different regions must unite to fight a common enemy.