Well, often there's a theme of transformation. The injection might be the start of some sort of change in the captive, either physical or psychological. It could also be about trust, or lack thereof. The captive has to deal with being injected by this handsome captor, and they may start to question whether they can trust him or not, which makes for an interesting plot development.
Create a strong and unique personality for the captive. If the captive is feisty or has some special abilities, the interaction during and after the injection becomes more engaging. For instance, if the captive has the power to heal but the injection is suppressing it, there's an immediate conflict. Also, you can introduce other characters who are trying to rescue the captive or are somehow involved in the captor's plan, which will add more layers to the story.
One potential consequence could be enhanced physical abilities. Since Hashirama cells are known for their powerful life force in the Naruto universe, Naruto might gain greater strength, speed, and stamina. He could also potentially have an easier time mastering certain jutsus related to wood release or other abilities associated with Hashirama.
One common theme is the power dynamic. There's a clear imbalance of power at the start, with the captor having control. Another theme is redemption. Often, the captor has some sort of past or actions that need to be redeemed in the eyes of the captured character as love develops. And of course, there's the theme of forbidden love, because falling in love with your captor goes against the normal social and moral norms.
Well, I'm not sure if you've heard of 'Love in Captivity'. It has a unique take on the 'falling in love with male captor' trope. The male captor in this story has his own reasons for capturing the main character, and as they are forced to be in close quarters, an unexpected bond forms between them. The story delves deep into their psychological states as their relationship progresses.
One great fanfiction could be where Naruto has mastered a new form that combines his good looks with an even greater power. Maybe he discovers a sage mode that enhances his natural charm while making him invincible in battle. This form could be a result of his training with a mysterious hermit who saw the potential in Naruto not just as a powerful shinobi but also as a symbol of hope and attractiveness to the people of the ninja world.
One great Naruto fanfiction where he's extremely handsome could be a story where he uses his charm to effortlessly win over allies from different villages. His good looks, combined with his kind heart and strong will, make him an even more appealing character, leading to new friendships and alliances that change the course of the ninja world.
A fanfiction idea could be that Saitama's hair growth is the result of some strange experiment. With his new handsome look, he accidentally becomes a model for a new line of superhero - inspired fashion. However, he still goes on his usual hero - hunting adventures, and his friends are both amused and a bit jealous of all the attention he's getting.
It could be Captain Hook. He has always been Peter Pan's nemesis and has a strong motive to capture him. Maybe he wants to finally get revenge for all the times Peter has outsmarted him.
There's a possibility that it's a villainous trainer. This trainer might capture Gengar with the intention of using it for evil purposes, like using Gengar's powers to steal other Pokémon or cause chaos in Pokémon battles.