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.
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 of the most powerful could be a fan - created character who has the ability to summon multiple Celestial Spirits at once. This would give them a huge advantage in battle as they could combine the powers of different spirits.
First, give your OC a clear backstory. If it's a celestial spirit, explain how it came to be. For example, was it created by a powerful wizard, or was it born from a special celestial event? Then, define its personality traits. Is it brave, shy, mischievous? These traits should be consistent throughout the story. Also, give it unique powers. Don't just copy the powers of existing celestial spirits. Make it something that stands out, like the ability to transform into different celestial forms depending on the situation.
Plue is quite popular. His cute appearance and simple yet endearing personality make him a favorite. In fanfiction, he might be given more complex emotions or be part of a big adventure that he has to bravely face.
One popular one could be 'The Hilarious Adventures in Fairy Tail Guild'. It has a series of short stories within the fanfiction that focus on the comical interactions between the guild members. Another is 'Fairy Tail's Funniest Pranks' which is all about the various pranks the characters play on each other.
You can try some popular fanfiction websites like FanFiction.net or Archive of Our Own. They usually have a large collection of 'Fairy Tail' fanfictions, including those about funny moments.