There could be a powerful demon from another dimension. As Natsu becomes an adult, his magic signature becomes stronger, which attracts this demon. The demon wants to possess Natsu's power for itself and will stop at nothing to achieve this goal. It would be a formidable foe, with abilities that Natsu has never faced before.
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 enemy of Natsu's resurfaces. This enemy is jealous of Natsu's newfound happiness with Cana and tries to take Cana hostage to force Natsu to fight him. This would create a major conflict in the fanfiction.
It could be another guild that sees Natsu as a threat. They might find out about Lucy kidnapping Natsu and try to use the situation to their advantage. For example, they could try to steal Natsu's magic or get information out of him while he's in a vulnerable state. Another possible antagonist could be a character who has a grudge against Lucy. They might try to stop her from getting away with the kidnapping or use it as a way to get revenge on her.
Perhaps in this fanfiction, when Natsu becomes an adult, he decides to explore different parts of the magical world that he was previously unable to reach. He could discover new types of magic or encounter different magical creatures. This could lead to him bringing back new knowledge to his guild, and they could use it to strengthen themselves. Also, his transformation might make him more of a leader figure within the guild, guiding the younger members.
A powerful dark guild could be the main antagonists. They might be jealous of the combined power of Natsu and Mirajane's family and seek to destroy them to gain more power in the magical world.
Maybe a rival guild that is jealous of Fairy Tail's new addition. They might think that having a baby with such strong magical potential in Fairy Tail gives them an unfair advantage. So they try to kidnap the baby to either keep it for themselves or use it to bargain with Fairy Tail.
The vampire Authority could be antagonists. They might see Sookie as an abomination since she was a fae - human before being turned into a vampire. They could try to eliminate her to maintain the 'purity' of the vampire race according to their own rules.
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.