They could become more like mother and daughter. The Fairy Godmother could teach Cinderella about her family history that she never knew. For example, that her real mother was a powerful magic - user too.
They could be given a task that requires them to rely on each other completely. Let's say they have to find a lost heirloom in a far - off land. Along the way, they share stories from their childhoods that they haven't told before. Rapunzel tells Flynn about her lonely days in the tower in more detail, and Flynn opens up about his early days as a thief. This deepens their bond.
Lucy could give Aquarius more freedom outside of the key. Let her experience the human world more often. They could go on adventures that are not related to Fairy Tail's main quests, just the two of them. This would allow them to get to know each other on a more personal level. Aquarius could also teach Lucy more about celestial magic and its deeper secrets, which would bring them closer as they explore this knowledge together.
They could have more deep conversations about their future as a family. For example, discussing how they want to raise their children, what kind of values they want to instill. Also, showing them supporting each other in small daily things, like Lucy helping Natsu with his reading skills and Natsu helping Lucy when she has a tough day at the guild. This would strengthen their bond on a more personal level.
A great way to develop their relationship further is to show them building a life together. They could start a project like building a lighthouse that has a special connection to both the land and the sea. It would be a symbol of their love and unity, and during the process of building it, they would face challenges, make compromises, and grow even more in love.
The relationship can be developed by having them face difficult challenges together. For example, in a tough Pokémon battle where they are outnumbered. They have to rely on each other's skills and instincts to win.
Their relationship could be developed through shared challenges. For instance, if they are both tasked with protecting a valuable item from being stolen by a new villain on the Isle. Working together to keep it safe would make them rely on each other more and strengthen their bond.
They could go on adventures together outside of Auradon and the Isle. This would put them in new situations where they have to rely on each other more, deepening their bond.
By showing them relying on each other's strengths more. For instance, Percy's water - related powers could be used in unique ways to protect Annabeth from the hellish fires of Tartarus, while Annabeth's strategic mind could help them navigate the complex and ever - changing landscape.
They could face a major conflict together, like a war between kingdoms. This would force them to rely on each other's strengths even more, Flynn's street - smart tactics and Rapunzel's kind heart and ability to connect with people. Their relationship would grow stronger as they fight side by side.
She could be made more sympathetic. Instead of being a villain, show her backstory of how she got into the business of trying to control others' lives. Maybe she was forced into it by some greater power, and in the fanfiction, she could break free from that control.