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.
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 a mission where Natsu and Laki are paired up by accident. Laki, being more of a support - type mage with her wood - based magic, could initially be intimidated by Natsu's brash and powerful fire magic. But as they face challenges together, she learns to stand up to him and they develop a mutual respect. For example, they might encounter a powerful earth - based monster that is immune to Natsu's fire. Laki's ability to manipulate the surrounding trees to entangle the monster gives Natsu the chance to figure out a new strategy to defeat it.
One idea could be that Natsu is exiled due to a false accusation of using dark magic. He then has to journey alone to prove his innocence, encountering new friends and foes along the way. Maybe he discovers a new form of magic during his exile.
One idea could be Natsu going on a solo mission to a far - off land that has a different type of magic entirely. He has to learn to adapt and control this new magic to complete his task. Another could be Natsu training a new generation of Fairy Tail wizards, passing on his unique brand of magic and combat skills.
One interesting plot idea could be Natsu going on a special quest with each member of his harem separately. This would allow for character development of both Natsu and the individual harem members. For example, when he's with Lucy, they could discover a new type of celestial spirit magic that only works in a specific, long - lost realm. Another idea is that an enemy targets Natsu's harem to get to him, so he has to protect them all while also dealing with his growing feelings for each of them.
In this fanfiction, Natsu could discover that his abandonment was part of a bigger plan within Fairy Tail to protect him from a greater threat. He might initially be angry and resentful, but as he uncovers the truth, he has to decide whether to forgive his old guild or not. He could also meet other characters who were also once abandoned and form a bond with them to face the challenges together.
One plot idea could be that Zeref discovers a way to reverse his curse through Natsu. Since Natsu is his creation, there might be some hidden connection that could be explored. Maybe Zeref tries to use Natsu's unique dragon - slayer magic to break the chains of his dark magic.
One idea could be Natsu crossing over into a modern-day world. He'd be confused by all the technology but use his magic in unexpected ways, like using fire magic to power up electrical devices. Another could be a crossover with a different anime world where Natsu has to team up with new characters to fight a common enemy. For example, in a world full of mecha, he might use his fire to overheat the enemy mechas.
A plot idea could be that an old enemy from Natsu's past returns and targets Lucy to get to Natsu. Natsu then has to go all out to protect Lucy, and in the process, they both learn more about each other's fears and insecurities. Or perhaps there is a magical competition in which Lucy and Natsu team up, and they face various challenges that test not only their magical abilities but also their friendship and trust in each other.
One idea could be that Natsu and Irene are forced to team up to defeat a powerful dark magic user. Their opposing personalities at first create conflict, but as they face challenges together, they learn to respect each other's powers.