Well, it could be about a variety of things. Maybe it focuses on the relationships between the characters after the main events of Fairy Tail. It might explore how the guild members' lives have changed, their new adventures, or even their personal growth.
Most likely, the main characters would be the core members of the Fairy Tail guild such as Natsu, Lucy, Gray, Erza, and Happy. They are the ones that the original story revolves around, so it's natural for them to be the focus in the fanfiction as well.
In the ending of Fairy Tail anime, the guild is restored to its former glory. The main characters like Lucy, Natsu, Gray, and Erza are all together. They defeat the final enemies and then return to a peaceful life in Magnolia. There are also some romantic hints between certain characters which add a touch of sweetness to the overall ending. It's a story that emphasizes the power of friendship and love in the face of great adversity.
It adds more content. The official Fairy Tail series has a limited number of episodes and chapters. Fanfiction fills in the gaps. For example, it can create stories about what the characters were doing during the time - jumps in the original story.
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.
One popular theme is alternate universes (AUs). Writers like to explore what would happen if the Fairy Tail characters were in different settings, like a modern - day high school or a steampunk world. Another common theme is romance. Fans often pair up their favorite characters in different relationships, such as Natsu and Lucy or Gray and Juvia.
Erza and Jellal are also a favored couple in Fairy Tail fanfiction. Their complex past, with Jellal's redemption arc, adds a layer of depth to their potential relationship. It allows for stories that deal with forgiveness, love that overcomes past wrongs, and the growth of their connection over time. There's also Gajeel and Levy, as their interactions in the series suggest a budding relationship that can be elaborated on in fanfics.
You can try websites like FanFiction.net or Archive of Our Own (AO3). FanFiction.net has a large collection of Fairy Tail fanfics. Just search for 'Fairy Tail' in the search bar, and you'll get a bunch of results. AO3 also has high - quality fan works, and it often has more in - depth and unique takes on the Fairy Tail universe.
The condition could be that the main character has to give up their magic for a period of time. This would create a lot of interesting plot twists as they'd have to rely on their wits and the help of their friends in the Fairy Tail guild instead of their magical abilities.