One main antagonist could be a rogue ninja from a neighboring village who is after the secrets of the Hyuga clan that Hanabi possesses. He sees Boruto as an obstacle in his way because of Boruto's growing reputation and power. This ninja is highly skilled in stealth and assassination techniques, making him a formidable foe for Hanabi and Boruto.
A group of outcast ninja who were once rejected by the Hidden Leaf could be antagonists. They blame the village's current generation, including Boruto, for their misfortunes. Hanabi gets involved because she wants to protect the village's future. These outcasts use dark and forbidden jutsu, which poses a great threat to Hanabi and Boruto as they try to stop them from causing chaos.
There could be an internal antagonist within the Hidden Leaf itself. A jealous ninja who feels that Boruto is getting too much attention and Hanabi is being too protective of him. This ninja starts to spread false rumors and create obstacles for them. Eventually, this jealousy turns into a more malicious intent, and they try to directly harm Hanabi and Boruto to get what they think is rightfully theirs.
A plot could revolve around a mysterious ninja technique that only Hanabi and Boruto seem to be able to master together. Hanabi, with her knowledge of the Hyuga clan's techniques, and Boruto, with his unique Rasengan variations, team up to figure it out. Along the way, they have to deal with other ninja clans who are jealous and try to stop them. This adventure brings them closer as they discover new things about each other and their own powers.
Maybe Mysterio. He could create illusions that make it difficult for Spiderman and Wasp to tell what's real and what's not. He might be trying to steal their powers or something related to their superhero identities. His ability to deceive would be a major obstacle for them to overcome.
A crisis situation could also be used to develop their relationship. Suppose there is an attack on the village and they have to protect it together. They would need to rely on each other's skills. Boruto's innovative ninja techniques combined with Hanabi's precise and powerful Hyuga jutsu could be a great combination. This shared experience of defending their home would make their relationship stronger and more meaningful.
Frieza and his minions would be obvious antagonists. They are the ones who brought so much destruction to the Saiyans. In a fanfic, they could be trying to finish off any remaining Saiyans, including the Saiyan princess and Bardock if he's still alive.
I'm not entirely sure specifically as 'lemon' in fanfic often refers to a type of fan - made story with romantic or sometimes more mature themes. But generally in relation to Boruto and Hanabi, it could be a fan - created story exploring a relationship between them in the Boruto universe, perhaps with some fictional and often uncanonical scenarios.
There are various 'Boruto x Hanabi' fanfics out there. A great one might explore their relationship in the context of the changing shinobi world. Maybe in a fanfic, Boruto is facing a new threat and Hanabi, with her knowledge of the Hidden Leaf's secrets, helps him out. It could also be a story about how they both deal with the legacy of their respective families. Hanabi being from the Hyuga clan and Boruto being Naruto's son. Their interactions in such a story could be really interesting, like how they bridge the gap between their family traditions and their own individual goals.
There could be an ancient evil force that has been dormant for a long time. The formation of the harem between Naruto and Moria Queen awakens this force as it senses the newfound power and unity. This antagonist would be extremely powerful and would pose a great threat to not only the harem members but also the entire world they are in, forcing them to work together to defeat it.
It could be a resurrected Orochimaru. He has always had an interest in power and the Akatsuki. He might return, more powerful than ever, and take control of the Akatsuki in the Boruto era, using them to carry out his evil plans, like creating an army of super - ninjas using some dark jutsu.
It could be a powerful sorcerer who wants to use Jason and Piper's powers for his own evil deeds. This sorcerer might try to manipulate them into doing his bidding or separate them to weaken their combined strength.
A jealous classmate could be a main antagonist. This person might be envious of Naruto's popularity and his ability to make friends easily. They could try to spread rumors or sabotage Naruto's projects.