In Fairy Tail fanfiction, how can the relationship between Natsu x Erza x Mirajane be developed in a unique way?

First, start with their friendship. They are both members of Fairy Tail, so they would interact often. They could share their love for the guild and gradually start to understand each other on a deeper level.

One possible storyline could be a quest where the three of them have to work together to defeat a powerful dark wizard. Natsu's brute strength, Erza's strategic combat skills, and Mirajane's shape - shifting abilities would make for an amazing team. They could start off with some initial bickering due to their different personalities but gradually grow to respect and rely on each other more as the adventure progresses.

One idea could be a story where they go on a quest together. Maybe they have to find a rare magical item. Natsu's impulsiveness, Mirajane's charm, and Erza's strict leadership combine in interesting ways during the adventure.

Their relationship can be developed through shared battles. For example, in a tough fight against a strong guild from another town, they rely on each other. Natsu protects Erza when she's in a vulnerable state during a requip, and Erza in turn uses her strategic mind to guide Natsu's brute force attacks. This mutual dependence can strengthen their bond.

Their relationship could develop through shared experiences. For instance, if they are both captured by a common enemy, they would have to rely on each other to escape. This would create a bond of trust between them.

Their relationship could be developed in a unique way if they had to swap magic powers for a short period. Gray having to use Erza's requip magic and Erza using Gray's ice - magic would force them to understand each other on a deeper level. They would have to train together to master the new powers, which would lead to a lot of close - quarter interactions and shared experiences.

The OC could be a character with a mysterious past. Maybe they have a hidden power that they are not fully aware of. Natsu, Mirajane, and Erza sense this mystery and are drawn to help the OC uncover their past. As they dig deeper, they find that the OC has a connection to an ancient magic that was thought to be lost. This discovery not only changes the OC's understanding of themselves but also has a significant impact on the relationships within the group.

A great way to develop their story is through a journey. Maybe they get sent on a quest to find a rare magical item. Along the way, they would encounter various obstacles. Mirajane could use her wisdom and experience to get them out of some tight spots, while Natsu's determination and strength would keep them going. This shared experience would bring them closer together in the fanfiction world.

Their relationship could develop realistically through shared experiences. For instance, if they both have to protect a young mage from a common enemy. This shared responsibility would force them to communicate more, understand each other's strengths and weaknesses, and gradually form a bond.

One great fanfic could be 'The Adventure of Natsu and Erza'. It focuses on their unique bond during a dangerous mission. They rely on each other's strengths and in the process, their relationship deepens in a very platonic yet heartwarming way.