It could be a powerful rogue ninja who has made Zacks Island his lair. He might be hoarding some valuable resources on the island and sees Naruto as a threat to his plans.
A group of bandits who have been terrorizing Zacks Island for a long time could be the main antagonists. They have a stronghold on the island and don't want Naruto to interfere with their operations. They are skilled in combat and use the island's terrain to their advantage, making it difficult for Naruto to take them on alone.
It could be a rogue ninja who has made Skull Island his base. He has found a way to control some of the dangerous creatures on the island and uses them to attack anyone who comes near. His goal is to find a powerful artifact on the island that he believes will make him invincible.
Well, a great plot for a 'Naruto Zacks Island' fanfic could involve Naruto being sent to Zacks Island for a special training mission. On the island, he discovers an ancient prophecy related to his Nine - Tails power. He has to work with the local inhabitants, who are initially skeptical of him, to fulfill this prophecy and in the process, he learns new techniques and gains a deeper understanding of his own power.
It could be one of the older boys who is power - hungry. He sees the girl as a threat to his leadership and tries to isolate her from the group. He spreads rumors about her and makes it difficult for her to be part of the decision - making process on the island.
It could be Orochimaru. He's always been interested in unique powers and abilities. The idea of X - 23's mutant abilities and Naruto's jinchuriki powers would be very appealing to him. He might try to capture them both to use in his experiments to gain ultimate power.
A new character could be created as the antagonist. For example, an Uchiha who has been corrupted by the power of the Sharingan and wants to use Naruto's special Uchiha - related abilities for his own evil purposes. This character could be jealous of Naruto's unique position within the Uchiha - related story and try to take it from him.
It could be a rogue ninja who has mastered parkour to an extreme level. He uses his skills to steal important ninja artifacts and Naruto has to stop him. This antagonist would be very fast and agile, making it difficult for Naruto to catch him using normal ninja techniques.
Madara could still be the main antagonist even in a 'naruto bankai fanfic'. He senses Naruto's new power and realizes it could be a threat to his plans of world domination. He adapts his strategies and attacks Naruto with all his might, trying to prevent Naruto from fully mastering and using his Bankai - inspired power to its fullest potential.
A powerful and corrupt ninja from another village could be the main antagonist. He hears about the unique knives in Naruto's village and decides to steal them to increase his own power and the dominance of his village. He would stop at nothing to get his hands on these knives, even if it means starting a war between the villages.
A mysterious figure within Konoha who has a hidden agenda. This person doesn't like the idea of Naruto breaking the norms and wearing a dress. They secretly manipulate events to make Naruto's life difficult while he's in the dress. For example, they might arrange for Naruto to be in situations where his dress - wearing causes maximum embarrassment, all in an attempt to make him conform to the traditional ways.
It could be Orochimaru. He has always been interested in the power of the dead and bringing back the strong. He might have found a way to reanimate Naruto for his own evil purposes, like using Naruto's power to take over the ninja world.