Who could be the main antagonist in a 'Scarlet Tides Fairy Tail fanfiction'?

It could be a former Fairy Tail member who turned to the gangster life. They know all the guild's secrets and weaknesses, so they can be a very formidable opponent. They might have left the guild because they felt betrayed or didn't get the recognition they thought they deserved.

It could be an ancient dragon that has been awakened from a long slumber. This dragon might have a vendetta against Ignia or the Fairy Tail guild for some past transgression.

One idea could be that a new character arrives in Magnolia during the Scarlet Tides event. This character has a mysterious power related to the tides and ends up joining Fairy Tail. They then have to work with the guild members to solve a mystery that is causing chaos during the tides.

A group of rogue mages who oppose the idea of Yukino joining Fairy Tail. They might view Fairy Tail as a threat to their own plans in the magical world. They target Yukino as the 'newcomer' in the guild, thinking that if they can get rid of her, they can weaken Fairy Tail. They use various underhanded tactics like spreading false rumors about Yukino within the guild and attacking her when she is on solo missions.

It could be Natsu. He's hot - headed and might not understand Naruto's ways at first. He could see Naruto as a threat to his position in the Fairy Tail guild and start bullying him to assert his dominance.

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.

Zeref could be a great main antagonist. His long - standing connection to dark magic and his complex relationship with the Fairy Tail characters make him a powerful and menacing figure. He could be the one orchestrating the universal war in an attempt to finally achieve his dark goals.

It could be a dark wizard who is jealous of Romeo and Wendy's emerging powers. This wizard might try to steal their magic or stop them from achieving their full potential.

The main antagonist could be a powerful demon that has been sealed in the Fairy Tail world for a long time. Naruto's arrival disrupts the seal somehow, and the demon sees Naruto as a key to fully breaking free. The Fairy Tail members and Naruto must put aside their differences and combine their powers to fight this demon.

A powerful dark wizard who wants to steal the Phoenix Slayer's power for himself could be a great antagonist. He could have an army of dark creatures at his command and constantly be devising plans to defeat the Phoenix Slayer and take over the magical world.