It could be Orochimaru. He has always been interested in Sasuke and might target ANBU Team 7 to get to him. Orochimaru's experiments and his desire for power make him a formidable and menacing villain. He could also be using some new jutsu or have created more powerful minions to fight against Team 7.
The Akatsuki could be the main villains. They are a group of powerful and dangerous ninjas. They might see ANBU Team 7 as a threat to their plans. The Akatsuki members have various unique abilities like Pain's ability to control multiple bodies or Itachi's powerful Sharingan techniques. Team 7 would have to use all their skills and teamwork to take on the Akatsuki.
Perhaps a rogue ANBU member who has turned against Konoha. This person has inside knowledge of ANBU operations and can use it to his advantage against Team 7. He might be seeking revenge for some past wrongs he believes Konoha has done to him. He could be very skilled in assassination and stealth techniques, making him a difficult opponent for Team 7.
Perhaps a powerful and unknown shinobi from a far - off land. This shinobi has heard of Team 7's reputation and sees them as a threat to his plans of world domination. He uses his unique and mysterious jutsu to challenge Team 7 at every turn, and they have to figure out his motives and weaknesses to stop him.
A new character could be created as the main villain. Maybe a rogue ANBU agent who was once a friend of Naruto and Sasuke but has been corrupted by power. This character would know all the ins and outs of the ANBU, making it very difficult for Naruto and Sasuke to stop him as he tries to take over the ninja villages using the secrets he learned while in the ANBU.
Perhaps a powerful ninja from another village who has a vendetta against Konoha and the ANBU. They see the ANBU's connection with Naruto as a threat and use their knowledge of the lemon fanfiction element in some way. For example, they might create clones that are disguised as lemon - scented ANBU members to infiltrate Konoha and cause chaos.
You might consider adding Tenten to the team. She has a vast knowledge of weapons which could be handy in various missions. And Kiba, with Akamaru by his side, could offer enhanced senses and tracking abilities for the Anbu team.
Perhaps an evil organization that wants to capture Ino for her unique mind - related abilities. They believe they can use her powers for their own malicious purposes. Naruto and the rest of the team then have to protect Ino and take on this organization. The leader of this group could be the main villain, with his own set of powerful and dark jutsu.
Maybe in the fanfiction, the ANBU Team 7 is sent on a mission to infiltrate a hidden organization that is planning to attack Konoha. Naruto uses his shadow clones in creative ways to gather intelligence. Sakura disguises herself as a member of the organization and uses her knowledge of poisons to take out guards silently. Sasuke uses his Sharingan to detect hidden traps and enemies. Along the way, they discover that there is a traitor within Konoha who is feeding information to the enemy.
One option for the main antagonist could be a secret organization that has infiltrated Konoha. They target the Anbu prodigy because they fear the prodigy's abilities could expose their existence and plans. This organization might have members with unique and dangerous abilities themselves. They would constantly send assassins after the prodigy and try to create chaos in Konoha to keep the Anbu and the prodigy distracted while they carry out their real objectives.
It could be an evil shinobi who wants to capture the Kitsune Anbu for their unique powers. Maybe they want to use those powers to take over the Hidden Leaf Village. Another option is a rogue Anbu member who has turned against the village and sees the Kitsune Anbu as a threat to their own plans.
It could be a rogue ANBU member who has gone bad and is trying to expose Naruto's secret identity for some evil purpose.
Sasuke could be one of the main characters. Maybe he and Naruto have a new kind of relationship now that Naruto has retired from the ANBU. They could be working together to protect the village in a different way.