Erza is a character full of potential. You could write about her adventures in other lands while on a solo mission, facing new challenges that test her strength and her ability to adapt to different situations outside of the Fairy Tail guild.
Gray is also popular. Fanfiction could expand on his backstory with Ur and how his ice - make magic could be developed in new and unexpected ways. Maybe he discovers a new form of ice - make that's even more powerful.
Natsu is a very popular character to focus on. His hot - headed and powerful nature can lead to all kinds of exciting storylines. Lucy is also a great choice. Her celestial spirit magic provides a lot of opportunities for different types of adventures, like exploring different spirit realms. And of course, Gray is popular too. His ice - make magic and his cool (pun intended) personality can be really fun to write about.
Erza is extremely popular. Her strong - willed and powerful nature means there are countless stories that can be told about her. Maybe she has to face an opponent who can nullify her magic, and she has to find a new way to fight. Juvia is also a fan - favorite. Stories about her unrequited love for Gray and her growth as a mage are always interesting. And of course, Happy, Natsu's sidekick, can add a lot of humor to any fanfiction.
Mavis is definitely a popular choice. Her unique powers and her role in founding the guild make her a central figure. Zeref is also often focused on, given his connection to Mavis and his powerful and mysterious nature. Then there's Precht, as he was an important part of the early guild and his actions had a significant impact on the future of 'Fairy Tail'.
Erza is often a popular character to focus on. Her strong will and past connection to the Tower of Heaven make her a central figure. Fans like to explore her growth during the events of the tower, as well as her emotional turmoil regarding Jellal. Another popular character is Jellal himself. His complex character, being both a villain and a victim in the Tower of Heaven arc, offers a lot of room for exploration in fanfiction. Gray is also a favorite. His cool - headedness during the battles in the tower and his relationship with his friends, especially Natsu, can be further developed in fan - created stories.
Natsu is always a great choice. His fiery personality and powerful dragon - slayer magic can create a lot of action - packed scenes. He's also very impulsive, which can lead to some interesting situations at the camp, like accidentally starting a small fire during a training exercise.
One idea could be to focus on an alternate origin story for a main character in Fairy Tail. For example, exploring how Natsu's past could have been different and how that would change his relationships in the guild.
Lucy and Levy are quite popular. Their friendship in the original series can be easily explored in a yuri context. Many fanfictions like to develop their relationship further into something more romantic.
Lucy is also a good character to focus on. She has a complex backstory with her family and the celestial spirits. You can explore her growth as a mage, how she becomes stronger in controlling her spirits and her relationships within the team. For example, her friendship with Natsu and how they support each other in battles. Plus, the whole aspect of the celestial spirit world is full of potential for interesting plotlines.
Natsu is often a popular character in these fanfictions. His energetic and fearless nature makes him a great mentor or big brother figure to an adopted character. He can teach them about the importance of friendship and determination in the world of Fairy Tail.
Obviously, Ace himself is a very popular character. His backstory from One Piece and his potential interactions with Fairy Tail characters make him interesting. Natsu is also popular in these fanfictions as he and Ace could have a great dynamic, given their similar personalities in terms of being hot - headed and powerful.