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.
The relationship can be developed through shared experiences. For example, if they are both involved in rescuing a kidnapped guild member. They'll face danger together, and Freed might use his magic to protect Lucy from harm, which will make Lucy see him in a new light. This will create a sense of trust between them.
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 Hibiki, his character could be developed by exploring his relationship with his guild more deeply in the context of his interactions with Lucy. Maybe he has some insecurities about not being as strong as some of the other guild members, and Lucy helps him see his own value through their adventures.
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.
One way their relationship could develop is through a mutual understanding of their powers. If they start training together to combine their celestial magic in new ways, they'll have to communicate and rely on each other more. This would not only strengthen their magical bond but also their personal relationship. Another aspect could be that they face a situation where they have to make sacrifices for each other. Say, Lucy has to give up a chance at a powerful magical item to save Loke, or Loke has to put himself in great danger to protect Lucy's family. These kinds of selfless acts would take their relationship to a new level.
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.
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.