Perhaps a rogue ninja from another village who sees Naruto as a threat. This ninja could target Anko and Naruto during their training. He might be jealous of the Hidden Leaf's strength and think that if he can stop Naruto from getting stronger under Anko's teaching, he can weaken the Leaf Village. He could be a skilled fighter with unique abilities that pose a real challenge to Naruto and Anko.
There could be an internal antagonist within the Hidden Leaf Village itself. Someone who doesn't like the idea of Anko teaching Naruto. Maybe this person is jealous of Anko or has some ulterior motives. They could try to undermine Anko's teaching by spreading false rumors about her or trying to get Naruto removed from her care. This would create conflict not only between Naruto and this antagonist but also between Anko and the village authorities if they start to believe the false rumors.
It could be Orochimaru. He might try to disrupt Tsunade's teachings to Naruto because he fears a more powerful Naruto under Tsunade's guidance. Orochimaru could send his minions to attack them during training sessions or steal scrolls that contain the knowledge Tsunade is about to teach Naruto.
It could be Orochimaru. He has always been interested in obtaining powerful jutsu and the Sharingan. If he catches wind of Itachi teaching Naruto, he might see Naruto as a new target to either steal power from or use as a pawn against Itachi and the Leaf Village.
Perhaps a strict teacher who doesn't like Naruto's unorthodox ways and Anko's rebellious attitude. The teacher might try to split them up and prevent them from participating in school activities together. For example, if they want to enter a school competition as a team, the teacher could create obstacles.
One possible plot could be that Anko focuses on teaching Naruto some of her unique ninja techniques. For example, she might teach him how to use snake - like jutsus or some special stealth skills she has mastered. This would enhance Naruto's combat abilities and give him an edge in future battles.
It could be Orochimaru. Since Anko has a history with Orochimaru, and Nariko being Anko's apprentice, Orochimaru might see Nariko as a threat or a tool to get to Anko. He could try to manipulate Nariko into doing his bidding or try to steal her newfound skills.
Perhaps an evil organization that wants to harness the power of both Naruto and Nero. They might see Naruto's chakra and Nero's Devil Bringer as the keys to ultimate power. This organization could be led by a powerful sorcerer who can manipulate dark magic and has minions from both worlds at his disposal.
It could be a rogue ninja from a hidden village who has a vendetta against Konoha. This ninja might target Kurenai and Anko specifically because of their skills and knowledge, and Naruto gets in the way while trying to protect them.
Maybe a new character created specifically for this fanfic. A powerful sorcerer who wants to use Naruto's chakra and Slade's combat expertise for his own evil purposes. He could be manipulating events from the shadows, pitting Naruto and Slade against each other at first until they realize they are being played.
It could be Darth Sidious. Since he is always looking for power and control in the Star Wars universe, he might see Naruto as a threat to his plans when Naruto arrives in Padme's world. He would try to eliminate Naruto and use Padme as a pawn in his political games.
It could be Orochimaru. He might be interested in Baby Beel's powers and try to capture him for his own evil experiments. Orochimaru would see Beel as a unique specimen and would stop at nothing to get his hands on him. Naruto would then have to use all his skills and the bond he has with Beel to keep him safe from Orochimaru.