Who might be the main antagonist in 'Naruto betrayed by Fairy Tail fanfiction'?

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.

Another possibility is that there is no single main antagonist. It could be a combination of misunderstandings, village politics, and personal issues within Kurenai herself that lead to her betraying Naruto. For instance, the village elders might be pressuring Kurenai to keep Naruto in check, and at the same time, Kurenai has her own doubts about Naruto's power which could be misconstrued as a reason to betray him.

It could be one of the harem members who has a hidden dark side. Maybe she's jealous of Naruto's closeness to others in the harem and decides to turn the rest against him, so she becomes the main antagonist.

In some cases, it could be a combination of characters. For example, a group of older and more traditional Fairy Tail members who don't like the new and different ways of the OC. They work together to isolate the OC, making them feel betrayed. So, this group of conservative guild members could be seen as the antagonists.

It could be Sasuke. He has had his own dark journey and might turn against Naruto and Ino in this fanfiction. His newfound power and different ideals could make him see Naruto and Ino as threats to his goals.

It could be Orochimaru. He's always been interested in Naruto and has his own devious plans. He might frame Naruto during the Chunin Exams to create chaos and further his own goals of obtaining power and knowledge. Orochimaru could use his henchmen or his jutsu to make it look like Naruto is involved in some sort of treachery.

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.

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.

It could be a dark wizard from the Fairy Tail world who sees Naruto as a threat because of his unique powers. This wizard might try to stop Naruto from leaving, fearing that he'll take the knowledge of Fairy Tail's magic to other places.

Maybe it's a new character created specifically for the fanfiction. This character could be a powerful sorcerer who has the ability to manipulate people's minds. He targets Goku out of jealousy for his power and popularity. By using his mind - control powers, he makes everyone around Goku believe that he's a traitor, thus making himself the main antagonist.