Well, an interesting antagonist could be a former Fairy Tail member who has gone rogue and now uses his knowledge of both land - based and sea - related magic to cause chaos in the ocean. He might have a grudge against Natsu specifically. Another antagonist could be a sea god who has been corrupted by the greed of humans. This sea god would try to punish all sailors, including Natsu, by creating dangerous storms and summoning sea monsters. And then there could be a shadowy organization that operates in the shadows of the sea ports. They are involved in illegal activities and see Natsu's presence as a threat to their operations.
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.
The main antagonists could include a powerful mermaid sorceress who has turned evil and wants to rule the seas. She might see Natsu as an obstacle to her plans because he represents the land - dwelling magic users. There could also be a group of sea bandits who are causing trouble for all the sailors in the area. They would target Natsu and his crew because they are known for their good deeds. Additionally, a sea - dwelling demon that has been awakened from a long slumber and is bent on destruction could be a major antagonist. It would be a formidable foe for Natsu as it has powers specific to the ocean environment.
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.
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.
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.
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.
Well, a cool plot could be that Natsu is on a mission to find a special treasure that is said to be hidden on a deserted island in the middle of the ocean. Along the way, he meets new friends who also have their own reasons for seeking the treasure. As they sail together, they face many challenges like storms and sea monsters. Also, Natsu might use his fire magic in creative ways to deal with the water - based obstacles, like evaporating water to create a path or using heat to scare away some of the sea creatures.
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.
I'm not sure specifically as it could be about various things. It might be a fan - created story where Natsu from Fairy Tail has some sort of adventure related to sailors. Maybe he meets sailor - themed characters or goes on a journey on a ship.
It could be Zeref. Since he has a complex relationship with Natsu in the original story, he might manipulate things behind the scenes to make the Fairy Tail guild betray Natsu. Maybe he wants to isolate Natsu for his own evil plans.
Perhaps an old enemy of Fairy Tail who sees Natsu as a new threat. They might target Natsu specifically to prevent him from strengthening the guild. This antagonist could have a long - standing grudge against Fairy Tail and see Natsu as the new hope for the guild, so they try to eliminate him before he can fully join.