Natsu could develop a sense of guilt. If he lost comrades during the war, he might blame himself for not being strong enough. This guilt could either drive him to become stronger or make him more withdrawn. He may also start to question his own beliefs. For instance, if he had to do some morally ambiguous things during the war to win, he'll have to grapple with the consequences of those actions.
He might become more brooding and distrustful. Banishment is a harsh experience, and Natsu, who was always so close to his Fairy Tail family, would feel betrayed. This could lead to him suppressing his emotions and becoming more solitary as he tries to figure out how to clear his name.
Her character development can be different in terms of relationships. As a female, she might be more empathetic towards other female characters in the guild, which could lead to different friendships. For example, she could form a closer bond with Lucy in a more sisterly way. In terms of her magical development, she may approach it with a different mindset. Maybe she's more cautious or strategic compared to the impulsive male Natsu. This could lead to her learning spells in a different order or mastering them in a different way.
He could return with a whole new set of allies. During the war, he might have met other powerful mages who decided to follow him back to Fairy Tail. These new allies could bring different types of magic and skills to the guild. Also, Natsu might come back with a different perspective on life. The horrors of war could make him more determined to protect peace in the world and in his guild, leading him to take on a more leadership - like role in Fairy Tail.
Their relationship might start off with a lot of pent - up emotions. Ash could be a bit distant at first due to his war experiences, but Serena's love and care would gradually draw him out. Eventually, they would become closer as they share their feelings and experiences during the time they were apart.
His character could become more conflicted. He has to balance his old values from Fairy Tail with the new, darker ones of the guild he joined.
At first, it might be a bit tense. Natsu would be full of questions and his future self might be a bit reluctant to share everything. But as they start to talk, they could become more understanding of each other.
Natsu's character development in this fanfiction might involve him becoming more responsible. With his end form, he has greater power and with that comes the need to use it wisely. He could start to think more about the consequences of his actions. For example, he might not rush into battles as blindly as he used to. Also, his emotional growth could be a part of it. He could become more in tune with his feelings towards his friends, especially Lucy. His loyalty to the guild could also deepen as he realizes his new form can be a great asset to protect Fairy Tail.
Often, Natsu becomes more confident. With the Dragon Force, he realizes his greater potential, which boosts his self - assurance.
Well, it could start with Natsu accidentally stumbling upon a time - related magic or a magical artifact that transports him to the time when his past self exists. Then they would meet and there would be a moment of shock and confusion.
Natsu would likely become more responsible. Taking care of a baby is no easy feat, and he'd have to learn things like being gentle and patient, which are not his usual strong suits. This would be a big step in his character growth.