If in an alternate universe, Mystogan was raised in a different guild and Natsu was sent to bring him back to Fairy Tail. Natsu would initially be frustrated with Mystogan's aloofness. However, as they travel together, Natsu would learn about Mystogan's past and the reasons for his detachment. This understanding could lead to a friendship developing between them, and in the end, Mystogan might decide to join Fairy Tail because of Natsu's influence.
Their relationship could develop through mutual respect for each other's strength. Natsu is a powerful wizard in Fairy Tail, and Kagura is no less in her own guild. When they first meet in battle or some other situation, they recognize the power in each other, which could be the start of a relationship where they gradually become friends.
At first, Natsu would probably be on guard around Irene. However, Irene's power and knowledge could be useful to Natsu in certain situations. As they spend more time together, Natsu's warm - hearted nature might start to thaw Irene's cold exterior. She could start to see him as more than just a Dragon Slayer and develop a soft spot for him. This could lead to a deepening friendship or a romantic relationship over time as they grow closer through shared experiences.
It could develop in a rather unexpected way. Suppose there's a magical accident that causes their minds to be linked for a short time. They get to see each other's deepest fears and desires. This experience could make them more empathetic towards each other. After that, Mystogan might be more protective of Wendy, and Wendy could be more understanding of Mystogan's often - mysterious ways. Their relationship could then evolve from a simple acquaintance in the guild to a pair that has a special connection that is unique to them.
Their relationship could develop deeper. After the wedding, they would start to understand each other on a new level. Maybe Natsu would become more responsible towards Lisanna.
Another way their relationship could develop is through a common goal that is not directly related to fighting. For instance, they both want to revive an old Fairy Tail tradition that has been forgotten. They work together to research, gather materials, and convince other guild members. During this process, they bond over their shared love for the guild and for the things that make Fairy Tail special, which in turn deepens their relationship.
In a fanfiction, their relationship might develop if they start to open up more about their pasts. Lucy could share more about her family and how she came to be a celestial wizard. Natsu could talk about his search for Igneel. By sharing these deep parts of themselves, they would build a stronger emotional connection. Also, small moments like Natsu saving Lucy from a small danger, like a falling object in the guild hall, can add up and gradually develop their relationship.
In fanfiction, their relationship might develop when they have to work together on a guild project. They start off not knowing much about each other but as they contribute their individual skills and ideas, they begin to understand and appreciate one another. For example, Minet's attention to detail could complement Natsu's more straightforward and impulsive approach. They learn from each other and this leads to a deeper connection.
Their relationship could develop differently if Natsu didn't initially see Zeref as an enemy. Instead of the typical animosity, Natsu could approach Zeref with curiosity. Maybe he wants to understand why Zeref created him and what his purpose really is. This could lead to a more cooperative relationship from the start.
Some fanfictions depict their relationship evolving as they participate in guild - wide events. For instance, during a festival or a celebration in the guild, they might have a conversation that reveals new aspects of their personalities to each other. Over time, these small interactions build up, and they become closer friends, or in some cases, the relationship might take on a more romantic undertone as the fanfiction writer chooses to explore that aspect.
One idea could be that Mystogan and Natsu have to team up to face a powerful dark wizard. Since Mystogan is usually more reserved and Natsu is hot - headed, their clashing personalities could lead to some humorous and intense moments during the fight.