One possible fanfiction could be about Erza having to face Siegrain in a battle where Siegrain is being controlled by some dark magic. Erza, with her strong will and powerful magic, tries to break the control on Siegrain while also fighting his attacks. This could show her internal struggle between her feelings for the person Siegrain once was and her duty to stop the danger he now poses.
A fanfiction could revolve around Erza and Siegrain in an alternate universe where they are allies from the start. They could go on adventures together, with Siegrain using his magic to complement Erza's skills. For example, Siegrain could use his magic to create illusions that help Erza in her battles, and in return, Erza protects Siegrain from physical threats. This would explore a different dynamic between them compared to the original story.
A plotline could be that Siegrain fakes his death and later reappears with a new identity. Erza discovers the truth and has to decide whether to trust him again or turn him in. This would create a lot of tension and drama as Erza wrestles with her emotions.
One idea could be that Erza and Kagura team up for a mission that involves an ancient magic artifact. They start off not getting along well because of their different guilds' rivalries in the past. But as they face various challenges during the mission, like powerful dark mages trying to steal the artifact from them, they gradually learn to respect each other's skills. They combine their combat styles, Erza's requip magic and Kagura's swordsmanship, to defeat the enemies and in the end, they become friends and vow to work together in the future for the greater good of the magical world.
In some fanfictions, Lucy and Erza could have a story where they swap abilities for a day. It would be really fun to see Erza trying to negotiate with the Celestial Spirits like Lucy does, and Lucy trying to handle the heavy armors and swords. This would lead to a lot of comical and also character - developing moments.
In some fanfictions where Erza dies, it could lead to a dark and tragic turn for the Fairy Tail guild. Her death might cause the other members to question their strength and purpose. For example, Natsu might be consumed by a desire for vengeance against the ones who caused her death.
One possible fanfiction idea could be that Erza decides to cut her hair as a symbol of a new beginning. Maybe she has just overcome a great battle or a personal struggle, and she wants to show that she is a changed person. The haircut could be a short, stylish bob that shocks her guildmates at first but then they grow to love it as it represents Erza's newfound confidence.
Perhaps the fanfiction could start with Jellal proposing to Erza in a very romantic way, like on top of a mountain overlooking a beautiful sunset. Then, the preparations for the wedding could be filled with some comical moments, like Natsu trying to help with the decorations but making a mess. After they get married, they might go on a honeymoon to a far - off land, where they encounter some old enemies who are now reformed and they all work together to solve a new mystery in that land. The fanfiction could end with them coming back to Fairy Tail stronger than ever and ready to face whatever comes next as a married couple.
It could involve Erza facing challenges in the Edolas version of Fairy Tail. Maybe she has to deal with different versions of her friends or new magical threats specific to that realm.
One possible reason could be a misunderstanding within the guild. Maybe some false information was spread about them, leading others in Fairy Tail to turn against them. Another reason might be a power struggle. If Natsu and Erza were becoming too powerful, some jealous members could have conspired to make it seem like they were betraying the guild. Or perhaps an outside force manipulated the situation to drive a wedge between them and the rest of Fairy Tail.
One possible plot could be that Natsu and Erza leave Fairy Tail due to some kind of internal conflict within the guild. Maybe there are differences in opinions regarding how the guild should be run or what kind of jobs they should take. So they decide to set out on their own journey to find a new purpose.
The way it fits into the 'Fairy Tail' universe. If the fanfiction can incorporate the magic, the guild's dynamics, and the overall world - building of 'Fairy Tail' while still focusing on Natsu and Erza's relationship, it becomes really interesting. For example, a fanfiction where the kiss is a result of a magical bond or a quest that they undertake together within the Fairy Tail world.