It could mean he's physically bulked up. Maybe he's been training really hard and has gained a lot of muscle mass, which makes him heavier. This would be a great development for his character as it would likely make him even more powerful in combat.
Perhaps the fanfic is exploring a different side of Natsu's character through this 'heavier' aspect. It could be related to an emotional burden he's carrying that is metaphorically making him 'heavier'. For example, he might have made a difficult decision or had a falling - out with a friend in the Fairy Tail guild. This internal struggle could be manifested physically in the story as him feeling heavier, and the author could be using this to add depth to his character development.
Well, it could mean that in a Fairy Tail fanfiction, the character Natsu is depicted as having feelings of hatred. Maybe the author of the fanfic has created a scenario where Natsu hates something specific like a particular character, a situation, or an event within the context of the Fairy Tail universe.
Erza would likely have a more serious take on it. If the fanfiction adheres to the values of Fairy Tail and portrays Natsu in a positive light that aligns with his character in the guild, she would probably approve. However, if it contains any inappropriate or un - Fairy Tail - like behavior, she would surely be against it.
It could be about Natsu having left Fairy Tail for some reason and then reuniting with the guild in a very unexpected way. Maybe he has new powers or a different outlook on life that causes some friction and also some amazing moments of reunion and growth within the guild.
One possibility is that it would bring a whole new dynamic to his relationships. His friends, like Gray, might be really confused at first. Natsu, as a girl, could potentially show a more delicate side while still having that fiery spirit. For example, in battles, she might use her strength in a more unexpected way, taking advantage of opponents underestimating her because she's a girl.
Natsu could end up teaming up with some unexpected allies from Tartaros. Maybe there are some members who have a change of heart and decide to help Natsu fight against the more evil elements within their own organization.
She could be a key figure in an ancient prophecy. For example, the prophecy might state that only the son of a certain dragon - slayer mother (Natsu's mother) would be able to defeat a great evil that threatens the Fairy Tail world. This would set Natsu on a path to discover his mother's identity and fulfill the prophecy.
One possible motive could be that he's been mind - controlled. There are many villains in the Fairy Tail universe who have the ability to control others. Another reason might be a misunderstanding. Maybe he thought Fairy Tail had betrayed him in some way. It could also be a result of some dark magic that has affected his judgment.
Well, one possibility is that Natsu could join Fairy Tail in a different way than in the original story. Maybe he has some special mission before officially becoming a member. For example, he has to prove his loyalty by helping an elder member with a personal problem.
One possibility could be that Natsu is attacked due to a misunderstanding within the Fairy Tail guild. Maybe a new member misinterprets Natsu's actions as a threat and attacks him in self - defense.