Well, perhaps Natsu is framed for stealing a powerful magical artifact. Someone could plant false evidence at the scene. Then he has to prove his innocence while also dealing with the distrust of his friends.
It could be a jealous rival from another guild. They might see Natsu as a threat and figure framing him would be a good way to get him out of the way.
It could be that Natsu is part of an experiment in a parallel universe within the fanfiction. The experiment gives him the ability to get pregnant. His journey would involve coming to terms with this new state, facing the challenges of pregnancy, and maybe even finding a way to reverse it if he wishes.
In a 'natsu gets married fanfiction', perhaps Natsu meets a new character who he falls deeply in love with at first sight. This new person might be from a different land. Natsu has to learn about her culture and traditions in order to marry her. His friends from the guild are initially skeptical but then come to support him wholeheartedly as they see how much he cares for this person.
Well, it could be that Natsu made a big mistake during a mission, like accidentally destroying a precious artifact that was crucial for the guild's future. So the guild master berates him for his carelessness.
In the 'natsu gets cana pregnant fanfiction', perhaps it starts with a magical accident. Natsu and Cana might be exposed to some strange magic that causes an unforeseen bond between them. As a result, Cana discovers she is pregnant. Then, they would have to face the reactions of their friends and the challenges that come with this new situation in their lives.
Well, it could involve a lot of emotional turmoil. Maybe Natsu initially panics but then slowly accepts his new role as a future father. They might face challenges from their friends or guildmates who are surprised by the sudden situation.
In such a fanfiction, perhaps Natsu would become extremely protective of Mira. They could also start to think about how to raise their child in a world full of magic and danger. Maybe they would seek advice from their more experienced friends like Makarov. Natsu might even start working harder to provide for the family, while Mira could focus on taking care of herself during the pregnancy.
He could marry an original character created specifically for the fanfiction. This character could be a powerful mage from a far - off place who crosses paths with Natsu during one of his adventures.
Well, it could be about Natsu being accidentally or deliberately left in a certain place. Maybe his friends go on a mission without him, and he has to find his own way back or prove himself while being left behind. It might also involve his feelings of abandonment and how he overcomes them.
I'm not sure specifically as I haven't read this particular fanfiction. However, it might involve a fictional scenario where Natsu and Ultear's relationship takes a turn towards having a family, with all the emotional and story - driven elements that come with it like how they deal with the pregnancy, how it affects their relationships with others in the story's universe.