A group of rogue ninjas from a hidden village could also be the villains. They might be after Nawaki because he's related to Tsunade, and they think they can use him as a bargaining chip. Naruto would then have to fight them off to save Nawaki. These rogues could have unique and dangerous jutsu that Naruto has to overcome.
There could be a new, mysterious villain who has a vendetta against the Senju clan. Nawaki, being a Senju, becomes a target. This villain might have powers that are unknown to Naruto at first, so Naruto has to figure out the villain's weaknesses while also protecting Nawaki from the various attacks.
One possible villain could be Orochimaru. He's always been interested in powerful ninja for his own experiments. Nawaki might have some special potential that Orochimaru wants to exploit, and Naruto has to stop him.
Perhaps Nawaki is in a situation where he's outnumbered by a group of rogue ninjas. Naruto, having heard of Nawaki's plight, races to the scene. He uses his shadow clone jutsu to create multiple clones and overwhelm the attackers. Naruto then takes Nawaki under his wing, mentoring him and sharing his own experiences as a ninja, which not only saves Nawaki physically but also helps him grow as a ninja.
The Akatsuki could be potential villains. They are always on the lookout for powerful beings or things that can help them achieve their goals. If the OC has something they want, like a special chakra or a unique ability, they would be a threat. Another could be rogue ninja from different villages who see the OC as an easy target or a means to get something valuable.
In a 'naruto saves korra fanfiction', the villains could be a combination of different forces. There might be rogue benders who have turned to the dark side, using their bending abilities for evil purposes. These benders could be working with some mysterious outsiders who bring in new and unknown threats. Maybe there are ancient evils that have been awakened and they see Korra as a major obstacle. Naruto would then have to face these complex and dangerous foes to save Korra.
Darkseid is another possibility. He's a powerful and menacing villain. If he sets his sights on Wonder Woman, Batman would be forced to act. He'd need to use all of his skills and resources to save her from Darkseid's Omega Beams and his army of minions.
One potential villain could be Orochimaru. He might have an interest in Naruto's sister for her unique chakra or some genetic trait. He could be using her as a pawn in his experiments to gain more power or knowledge about the Uzumaki clan.
In the 'Naruto Saves Luffy and Ace Fanfiction', the main villains might include the World Government from One Piece. They have a lot of power and resources and are often against pirates. Also, there could be some rogue ninjas from Naruto's universe who are attracted by the chaos and want to take advantage of the situation. These villains would pose a great threat to Naruto's mission of saving Luffy and Ace, making the story full of tension and conflict.
Well, obviously Nawaki himself is a main character in most 'naruto nawaki fanfiction'. Alongside him, Tsunade often features prominently as his sister. Sometimes Naruto can also be a main character, especially if the fanfiction involves Nawaki's influence on Naruto's journey or if they interact in some way, like Nawaki becoming a mentor - like figure to Naruto.
A common villain could be Orochimaru. Since he was always interested in powerful jutsus and abilities, he might target Sakura to use her as a pawn in his experiments. And Naruto, with his demon powers, would have to stop him. Orochimaru is a well - known and menacing character, so it makes sense for him to be a villain in these fanfictions.
In this fanfiction, it's possible that an ancient and powerful beast is the antagonist. The beast might have captured Pyraa to lure Naruto into a trap. Naruto will need to use all of his skills, including his Sage Mode if necessary, to defeat this beast and save Pyraa.
One possible antagonist could be the Akatsuki members. They are always looking for strong opponents or valuable information. Maybe they target Iruka because they think he has some knowledge about Naruto that they can use, and Kakashi has to stop them.