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.
Maybe a dark wizard who has a vendetta against Natsu's guild. Framing Natsu would not only cause trouble for him but also for the whole guild. The dark wizard could use his dark magic to create false evidence and manipulate the situation to make it look like Natsu is guilty.
There's also a possibility that it's an internal betrayal. A disgruntled member of Natsu's own guild who has some hidden motives could frame him. This would add an extra layer of drama as it's someone close to Natsu who is causing him this trouble.
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.
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.
It could be Gray. Although they are friends most of the time, in a fanfiction, there could be a plot twist where Gray's envy of Natsu's abilities gets the better of him, and he betrays Natsu. Maybe Gray thinks that by getting rid of Natsu, he can be the top mage in their guild.
One of the senior and more experienced members of the guild. They have seen a lot and know how things should be done in the guild. If Natsu acts in a way that goes against their long - established ways, they would not hesitate to berate him.
It could be a dark wizard who has a vendetta against Fairy Tail. Maybe someone who Natsu defeated in the past and is now seeking revenge.
It could be a young, inexperienced mage who has always admired Natsu from afar. Maybe someone with a hidden power that Natsu senses and decides to take under his wing.
It could be a long - lost relative of an old enemy. For example, if there was a powerful dark wizard that Natsu defeated in the past, this relative might come seeking revenge and be powerful enough to kill Natsu.
One possible reason could be that Erza might have taken on a dangerous mission alone without informing Natsu. Natsu is protective of his friends and would get angry if he thought Erza was putting herself in unnecessary danger.
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 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.