A group of dark mages who are against the idea of an alpha in a guild like Fairy Tail. They see it as a threat to their own power - hungry plans. They could constantly plot to eliminate Natsu and disrupt the harmony of the Fairy Tail guild.
Maybe an ancient evil that has been awakened by Natsu's growing power as an alpha. This entity could have a vendetta against the Fairy Tail guild from long ago and sees Natsu as the key to either its destruction or its resurrection, and thus becomes a major antagonist.
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.
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.
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.
One idea could be Natsu leading a special mission within the guild that no one else has been able to complete. Maybe it involves a powerful dark magic that only his alpha - like determination can overcome.
The dark guilds would be great antagonists. They might see Natsu's new leadership as a threat and try to undermine his guild. They could send their best mages to disrupt the guild's operations and steal important magical artifacts.
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.
Well, in fanfiction where Natsu leaves the guild, he could become a sort of lone wolf. He might travel to far - off lands to discover more about his dragon - slayer powers or his past. This could lead to him meeting new and powerful allies or enemies. Without the support of the guild, he would have to rely solely on his own strength and wits. It could also make him a more serious and hardened character, as he no longer has the safety net of his Fairy Tail family.
It could be a powerful dark wizard who sees Natsu as a threat because of his alpha status among dragon slayers. This wizard might want to eliminate Natsu to gain control over the dragon slayers or to carry out some evil plan without interference.