In Fairy Tail celestial spirits fanfiction, writers get creative with the powers and personalities of the spirits. Some may give a spirit a backstory that wasn't explored in the original. For instance, Aquarius could have a past that involves an ancient water civilization, which adds more depth to her character and her relationship with Lucy.
Another good one is 'Celestial Bonds Renewed'. This fanfiction explores the idea of the celestial spirits' bonds with their summoners being tested in new and unexpected ways. It's full of emotional moments between the characters, especially Lucy and her spirits.
One idea could be creating an OC celestial spirit that has a unique power related to time. For example, it can freeze time briefly in a small area. Another idea is an OC that has a special bond with a main character from Fairy Tail, like Lucy. Maybe the OC celestial spirit was once a human who was transformed into a spirit due to a powerful magic spell long ago. Also, you could write about an OC celestial spirit who has a conflict with the existing celestial spirit system and tries to change it for the better.
One idea could be exploring the backstories of the Celestial Spirits during the Eclipse event. Maybe some of them have hidden powers or relationships that are only revealed during this time.
Lucy's new power would bring a shift in her relationships. With Happy, he might be constantly curious about her Celestial Dragon Slayer magic and pester her with questions. The guild as a whole would view her in a new light. Some might be jealous of her newfound power, while others, like Levy, would be excited to research more about it. This could lead to new friendships forming as Lucy shares her experiences with others in Fairy Tail, and also cause some minor frictions as she has to navigate through the different reactions to her power.
I'm not really into this kind of 'forced' concept in fanfiction. However, if we analyze it in a general sense, it might be a very dark and unethical take on the relationship between Lucy and the celestial spirits. It goes against the normal friendly or contractual relationships depicted in the original work.
In some fanfictions, Lucy becomes powerful by forging a unique bond with all her spirits simultaneously. This bond enables her to combine their powers in unexpected ways. She could create a powerful shield made of starlight by having her spirits channel their energy through her. This not only protects her but also can be used as an offensive weapon when she reflects the shield's energy towards her enemies.
If Lucy never joined Fairy Tail, the friendships and relationships among the characters would form in a very different way. Also, many of the story arcs might not unfold as they do in the original plot.
You can start by combining the traits of different Celestial Spirits. For example, take the loyalty of one spirit and the mischievousness of another to create a new and interesting character.
One popular theme is Lucy's growth as a wizard. Fans often explore how she overcomes new challenges and becomes stronger. Another theme is her relationships, not just romantic ones but also her friendships within the guild. For example, her bond with Natsu and how they support each other through tough times.