Perhaps a new character could be the main antagonist. This new character could be a powerful ninja from another village who has some sort of connection to Konoha. They might be influencing Tsunade's decisions in a negative way, for example, by making deals that Naruto sees as harmful to Konoha. Naruto snaps at Tsunade because he doesn't understand why she is going along with this new character's demands. This new character then becomes the main source of conflict as Naruto tries to expose their true intentions and fix the rift between him and Tsunade.
Well, Naruto himself could be involved. He might be concerned about Tsunade's pregnancy and offer his support in some way, like running errands for her or helping to keep the village in order while she's dealing with her pregnancy.
Tsunade in Tsunade lemon fanfics typically maintains her tough exterior that she is known for in the Naruto series. However, in the context of these fan - created stories, she can also be shown as having a softer side when it comes to her romantic relationships. Her beauty and maturity are often highlighted, and she might be involved in complex emotional situations. For example, she could be in a relationship where she has to balance her duties as a leader and her feelings for a particular character, all while dealing with the 'lemon' aspects of the story which might involve passionate encounters or deep emotional connections.
There might also be other characters involved, like Sakura. Since she was Tsunade's apprentice, she could be part of the story. Maybe she's caught in the middle when Tsunade has to take some action regarding Naruto. Also, Kakashi could be in the fanfic. He has a close relationship with Naruto and might be affected by whatever Tsunade does to Naruto or be involved in the events that lead to Tsunade's actions.
It could be a group of rogue ninjas who also ended up on the island. They might be after some valuable resource on the island and see Naruto and Tsunade as obstacles.
It could be a group of rogue ninjas who want to steal Naruto's godlike powers for themselves. They might be led by a powerful and evil ninja who has his own motives for wanting such power.
Jiraiya is also sometimes considered a close friend in fanfics. Given their shared history as part of the Sannin, there are fanfics that explore their complex relationship. They may have had adventures together in their youth and still hold a special place for each other in their hearts.
Tsunade could interact with Midoriya Izuku. Izuku is always eager to learn new things and improve his skills. Tsunade could see his determination and potential, and she might teach him some of her combat and medical knowledge. Maybe she would help him control his quirk better or give him some physical training tips.
I think all three are great in their own ways. Naruto is the heart of the story, his optimism and determination are infectious. Sakura has the potential to be a complex and strong female character. She can have her own character arc that is independent yet related to Naruto and Tsunade. Tsunade, with her long history and powerful abilities, can be a source of both conflict and resolution in the fanfictions. For example, her past experiences can create interesting plot twists when combined with the current situations of Sakura and Naruto.
It's Naruto and Tsunade. Naruto is known for his determination and his ability to make friends and influence those around him. Tsunade, on the other hand, is a strong - willed woman with great medical and combat skills. In this fanfiction, their interactions are likely to be the focus, with Naruto looking up to Tsunade as a mentor and Tsunade seeing Naruto as a symbol of hope for the future of the village.