Natsu would also be an interesting choice. Natsu's brash and impulsive nature could contrast well with Mystogan's more reserved and calculated personality. They could start off not getting along but then grow to respect each other during a difficult quest.
There could be a fanfiction where Mystogan has a premonition about a future threat to Wendy. He then decides to secretly protect her. Along the way, Wendy discovers what he's doing and they have a heart - to - heart. This could lead to a development of romantic feelings between them, or at least a very strong platonic bond as they face the impending danger together.
A plot could involve Mystogan's past coming back to haunt him. An old enemy from his homeland tracks him down in Fiore. This forces him to face his past mistakes and traumas. Alongside his Fairy Tail friends, he has to find a way to defeat this enemy and finally come to terms with his past.
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.
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.
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.
In this fanfiction, perhaps Natsu's return as a 'king' means he has a new level of leadership. He might bring back some lost magic or knowledge that helps Fairy Tail regain its former glory. For example, he could have found a way to defeat an old enemy that was once too powerful for the guild. His return could also stir up emotions among the guild members, like excitement, relief, or even some jealousy from those who might have risen in his absence.
Well, one interesting idea could be the return of the dragons bringing new powers to the Fairy Tail wizards. Maybe they bestow unique magical abilities that help the guild face even greater threats. Another idea might be the re - establishment of old bonds between the dragon slayers and their dragon parents or mentors, leading to some really heart - warming or intense emotional moments.
Natsu is the central figure in this fanfiction. But other characters like Happy, his loyal flying cat companion, will surely play an important role. Makarov, the guild master, would also be a main character as he has to deal with Natsu's return, whether it's a welcome or a bit of a shock to the guild's current state. And don't forget about the villains or antagonists that Natsu and Fairy Tail might face upon his return, they could also be considered main characters in the sense that they drive the plot forward.
One interesting concept could be the return of the dragons and how it affects the power dynamics in the Fairy Tail world. Their return might bring new challenges and opportunities for the characters to grow.