In some 'natsu returns to fairy tail fanfiction', a great plotline is Natsu returning with a secret mission. Maybe he has to protect Fairy Tail from a new and unseen threat that he discovered during his absence. His return then becomes a race against time to prepare the guild for this danger. This can lead to a lot of action and suspense within the fanfiction.
One interesting plotline could be Natsu coming back with new and powerful magic. For example, he might have learned some ancient spells during his time away. Another could be his reunions with the various members of Fairy Tail. He would have emotional moments with Lucy, Happy, and the others, reminiscing about old times and looking forward to new adventures together.
One interesting plotline could be Natsu and Juvia teaming up for a difficult mission. They have different magic styles, Natsu with his fire magic and Juvia with her water magic. Their interaction during the mission, like Juvia being a bit shy around Natsu at first but then growing more confident as they face challenges together, can make for a great story.
One interesting plotline could be Natsu discovering new dragon powers that are unique to him. This could lead to him having to learn how to control these powers and facing challenges within the Fairy Tail guild as others may be wary or envious of his new abilities.
One possible plotline could be Natsu turning dark due to some external dark magic influence and his journey to regain his former self while away from Fairy Tail. Maybe he teams up with some unexpected allies.
One common plotline could be Natsu going on a solo adventure that the other Fairy Tail members watch through some magical means. Maybe he discovers a new type of magic that is both dangerous and alluring. Another could be Natsu facing a version of himself from an alternate timeline, and the guild watches as he battles his other self to save the world.
A lot of fanfictions focus on the idea of Natsu and Lucy being forced to work together in a tournament. They have to overcome their differences and use their combined skills to win. This often leads to a better understanding of each other. There are also plotlines where Lucy discovers a new and powerful form of her celestial magic and Natsu helps her master it. Sometimes, a prophecy about their combined destiny is revealed, which sets them on a journey to fulfill it.
One unique plotline is where Juvia and Natsu accidentally swap magic powers. This leads to a lot of chaos as Juvia tries to control Natsu's fire magic and Natsu struggles with Juvia's water magic. They have to learn to adapt quickly to this new situation while also dealing with the threats that come their way.
One popular plotline could be Brandish and Natsu teaming up against a powerful common enemy. They might start off distrusting each other at first, but as the story progresses, they learn to respect and rely on one another. Another could be a story where Brandish has some sort of magic problem and Natsu, with his unique fire magic, helps her solve it. It could also be a story set in an alternate universe where they are on a quest together to find a powerful magical artifact that could change the fate of the Fairy Tail guild and Brandish's own people.
One popular plotline could be Millianna and Natsu teaming up for a difficult mission. Maybe they have to infiltrate an enemy stronghold. Millianna's cat - like abilities and Natsu's fire power would make for an interesting combination. They could face challenges that require both stealth and brute force.
One possible plotline could be that Lucy uses her charm and intelligence to plan elaborate schemes to get Natsu's attention. For example, she might invite him on special adventures that are just for the two of them, where she can show off her skills and bond with him more deeply. Another could be that she gives Natsu special gifts that are meaningful to him, like a scarf she made herself which has some magical protection. Or perhaps she starts dressing in a way that she knows Natsu will find appealing, not in a vulgar way but in a way that highlights her beauty and confidence.
One possible plotline could be Natsu trying to prove his innocence. Maybe he was wrongly accused of a serious crime within the Fairy Tail guild, like endangering the lives of other guild members during a mission. He then sets out on a journey to find evidence that clears his name while also facing various challenges and enemies along the way.