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.
Their friendship can be developed by having them face a really tough challenge together. For example, if they are trapped in a dangerous magical maze. They would have to rely on each other's strengths. Lucy's intelligence and Aquarius' power could be combined to find a way out. This would deepen their bond.
Lucy could be developed further by exploring her family background more deeply. Maybe she discovers some long - lost relatives who are also involved in magic. This could add new layers to her character. Another way is to show her struggling with her insecurities about her magic abilities and then gradually overcoming them through various challenges.
One idea could be that Aquarius gets trapped in a different dimension, and Lucy has to go on a dangerous quest to rescue her. She might have to team up with some unexpected allies from Fairy Tail. Another plot could involve Aquarius losing her memory, and Lucy has to help her regain it while also protecting her from those who would take advantage of her in this vulnerable state.
Well, a plot idea might be that a powerful wizard wants to steal Aquarius' key from Lucy. Lucy has to fight to keep it. Along the way, she discovers that Aquarius has some hidden abilities that she didn't know about before. Another idea is that there is a prophecy about Lucy and Aquarius that they must fulfill together. They initially don't believe it but as events unfold, they realize they have to work as a team to save the world.
For Lucy, her character could be developed by showing her learning new and advanced celestial magic spells from Yukino. This would not only expand her magical abilities but also show her determination to grow stronger. For Yukino, she could be developed by showing her becoming more confident in making decisions outside of her guild's rules, influenced by Lucy's more independent nature.
They could have flashbacks during the wedding planning of all the times they've saved each other. This would deepen their connection and show how far they've come. For example, when Natsu saved Lucy from the Phantom Lord attack.
For Lucy, you could explore her celestial spirit powers more deeply. Maybe she discovers a new and rare spirit that has a special connection to Rogue or Sting's magic. For Rogue, his relationship with his shadow could be further developed. He might learn to use his shadow in new ways, and Lucy and Sting could play a role in that discovery. For Sting, his leadership skills could be tested and improved. They could be put in a situation where Sting has to lead a group that includes Lucy and Rogue, and he has to make tough decisions.
Lucy could be developed by showing her struggle to control the more powerful star dresses. For example, she might initially be overwhelmed by the strength of a new star dress but gradually learn to master it through training and self - discovery. This would show her growth as a mage.
To develop their relationship, start with small interactions. Have the OC offer to help Lucy with her celestial spirit summoning, like finding a rare ingredient for a spirit - summoning spell. Then, they could have some shared experiences, such as getting trapped in a cave during a mission. This forces them to rely on each other, talk, and get to know one another better. Another way is to give them common goals. For example, they both want to defeat a certain dark mage who has been causing trouble in the area. Working towards this goal together will bring them closer.
Their relationship can be developed by having them share secrets. For example, Aslan could reveal to Lucy some of the deepest mysteries of Narnia that no one else knows. This would create a special bond between them.