The Fairy Tail guild members who supported Natsu's exile could be antagonists. They might believe in false accusations against him or have their own motives for wanting him out of the guild, like jealousy of his power or popularity.
Perhaps an old enemy of Natsu's from his past resurfaces during his exile. This enemy could have been biding their time, waiting for an opportunity when Natsu is alone and without the full support of Fairy Tail. They might try to get revenge on Natsu for past defeats or use his exile as a chance to cause chaos in the magical world.
It could be a group of students from another academy who are jealous of Fairy Tail Academy's reputation. They constantly try to undermine Natsu and Lucy's efforts and cause trouble during competitions.
It could be a group of rogue mages who have a vendetta against Natsu and Erza. They see the two leaving Fairy Tail as an opportunity to attack them when they are more vulnerable outside the protection of the guild.
One possible antagonist could be a dark wizard who wants to control the sea for his own evil purposes. Maybe he plans to use the power of the ocean to spread his dark magic across the land. Another could be a sea pirate captain who is jealous of Natsu's power and tries to stop him at every turn. Or it could be a sea monster that has been corrupted by dark magic and sees Natsu as a threat.
One possible antagonist could be a dark wizard from a rival guild who is jealous of Natsu's status as an alpha in Fairy Tail. This wizard might try to undermine Natsu's leadership and the guild's reputation.
It could be a group of jealous students who don't like Natsu and Lucy being in charge. They might try to undermine their leadership by causing trouble during important events at the academy.
One idea could be that Natsu is exiled due to a false accusation of using dark magic. He then has to journey alone to prove his innocence, encountering new friends and foes along the way. Maybe he discovers a new form of magic during his exile.
It could also be a story about Natsu's personal growth during his exile. Without the support of his Fairy Tail friends initially, he has to rely on his own strength and wits. He might discover new magical abilities or face challenges that test his character, and in the end, he either returns to Fairy Tail as a changed and stronger wizard or decides that his new path outside the guild is better for him.
A group of dark fairies could be the main antagonists. They might be jealous of Natsu's power or his unique status as a fairy and try to undermine him at every turn.
Maybe an old acquaintance from Natsu's past that we haven't seen much in the original Fairy Tail story. This person could be a powerful but reclusive mage who decides to help Natsu out of respect for his father or some past debt. They could offer Natsu shelter and train him in new forms of magic to help him face the challenges during his exile.
In fanfiction, Natsu's age can vary greatly. Some authors might keep him at his canonical age from the original Fairy Tail series, which is around 17 - 18 years old at the start. However, in alternate universe fanfictions, he could be older or younger depending on the plot. For example, in a time - travel based fanfic, he could be much older if the story takes place far into the future or much younger if it goes back to his childhood days.