Well, it could be about the guild rebuilding and mending the relationships that were strained during the Tartarus arc. Maybe some characters have new powers or a deeper understanding of their magic.
In 'After Tartarus' fanfiction, it might involve new villains emerging who are either remnants of Tartarus or new threats attracted by the weakened state of Fairy Tail after the Tartarus battles. The story could then revolve around the guild's efforts to not only deal with these new enemies but also to regain its former glory. There could be sub - plots about individual characters' growth, like Lucy becoming a more powerful celestial wizard by forging new contracts or Erza learning to better control her emotions during combat to avoid making hasty decisions.
One idea could be exploring the backstories of the Tartarus members in more depth. For example, how they came to join Tartarus and what their initial motives were. Another could be a story where the Fairy Tail members have to infiltrate Tartarus in a very different way, like using magic that disguises them as Tartarus members.
One interesting Tartarus Fairy Tail fanfiction could be about the characters' journey to overcome the dark powers of Tartarus in a different way than in the original story. For example, a story where they have to form unexpected alliances within Tartarus itself.
Natsu is often very popular. His fiery personality and powerful magic make him a central figure in many fanfictions. He's likely to be the one leading the charge against Tartarus, using his dragon slayer magic in creative ways to combat the dark forces.
Natsu is definitely a popular choice. His powerful fire - dragon slayer magic makes for exciting battles against Tartarus foes. Also, Gray with his ice - make magic can create some really cool and intense fight scenes. Erza is another one. Her ability to change armors and her strong combat skills are great for a 'Fairy Tail Tartarus' fanfiction.
Lucy could also be the main character. She has a lot of room for growth and development. Maybe in the 'After Tartarus' fanfiction, she discovers a new celestial spirit that has a connection to Tartarus, and she has to figure out how to deal with it while also protecting her friends in the guild.
It could be about Natsu having left Fairy Tail for some reason and then reuniting with the guild in a very unexpected way. Maybe he has new powers or a different outlook on life that causes some friction and also some amazing moments of reunion and growth within the guild.
It can introduce a major conflict. If Erza is corrupted, it's like a bomb in the middle of Fairy Tail. Her actions as a corrupted character can disrupt the normal harmony of the guild. For example, she might oppose her friends or engage in actions that are harmful to the guild's reputation. This then forces the other characters to deal with this new threat, changing the direction of the overall plot.
Well, one possible plot could be that Goku somehow ends up in the Fairy Tail world. He might initially be confused by the magic there, but then start to learn about it. He could team up with Natsu and the gang to fight against a powerful dark wizard. Goku's strength and his unique fighting skills like the Kamehameha could be a great asset to Fairy Tail in battles.
Well, in this fanfiction, Igneel's return could bring a huge shift in power dynamics. His relationship with Natsu might be explored more deeply, like they could have some intense training sessions. Also, it might lead to new quests for the Fairy Tail guild as Igneel's knowledge could be used to combat stronger enemies.
One idea could be that Aries discovers a new type of celestial magic that she has to keep secret from the other celestial spirits. She struggles with whether to tell Lucy or not, as it could put her in danger. Another plot could involve Aries getting lost in a different dimension while on a mission with Lucy and having to find her way back, all the while learning more about her own powers and past.