It's hard to really say positive themes for this kind of fanfic as it's a rather odd and not - so - appropriate concept. In general, Naruto is about things like loyalty, perseverance, and the pursuit of one's dreams. This kind of fanfic seems to be more about creating shock value or exploring some very specific, and often not - in - line - with - the - original - series sexualized ideas.
Maybe some would try to use it as a way to show female empowerment in the Naruto world, but it's a very strange and inappropriate way to do so. The Naruto series has its own ways of showing strong female characters without having to rely on such fanfic themes. For example, characters like Tsunade are shown as powerful leaders and ninjas in the official story.
Most official Naruto media focuses on the battles, character development, and the overarching story of the ninja world. Fanfics that involve the 'Sexy Jutsu' in a lesbian context are likely to be a very niche and perhaps not very well - received by the general Naruto fan base as it can be seen as objectifying characters in a way that doesn't fit with the core values of the series.
It could be a new form of Rasengan. Maybe Naruto discovers a way to combine different elements with the Rasengan more effectively, like adding lightning to it for a more powerful and faster attack.
One common theme is the development of relationships. Since it's a harem, Naruto has to build different kinds of relationships with multiple characters. Another theme is the use of jutsu in a creative way to attract or interact with the harem members.
One common theme is the discovery of new sealing jutsu. Writers often create new and powerful sealing techniques for their characters to find or develop. Another theme is the power struggle related to sealing jutsu. Since they can be used to seal away great power or even demons, there's often a fight over who controls them.
Naruto might improve the Sexy Jutsu by using chakra in a more creative way. He could use it to create an illusion of a more alluring environment around him during the jutsu, like a beautiful garden or a luxurious room. This would enhance the overall effect of the jutsu.
Well, in the Naruto universe, for Naruto to make a Treant Jutsu, he might first need to study nature chakra deeply. He could observe how trees grow and thrive, and then try to infuse his own chakra with the essence of nature. Maybe he'd have to combine his shadow clone jutsu in a new way, having clones study different aspects of trees, like their root systems and how they absorb nutrients. Then he could use his Rasengan as a base and add the nature - based tree - like elements to it, gradually forming a Treant - like structure out of his chakra.
He might improve it by making the transformation more dynamic. Instead of just a static form, it could have special effects like a glittery aura or flowing hair that seems to move on its own. Also, Naruto could experiment with different voices during the jutsu to make it more seductive.
In 'naruto lesbian fanfic', a frequent theme is the rediscovery of self. Female characters might discover new aspects of their personalities through their relationships. For instance, a shy character could become more confident because of the love and support of her female partner. Also, there's often the theme of defying the norms of the ninja world. In the original Naruto, relationships were mostly hetero - normative, but in these fanfics, characters break those boundaries and create their own unique love stories.
In fanfiction, it could be that the Sexy Jutsu has a hidden power. Maybe it emits a certain type of chakra wave that affects the Third Hokage's senses. Since he's an older ninja, his senses might not be as sharp when it comes to dealing with such an unorthodox jutsu. So, Naruto's Sexy Jutsu can disrupt his balance and make him vulnerable to Naruto's attacks.
One popular type might be those that explore how his opponents react when he uses the Sexy Jutsu in unexpected situations. For example, in some fanfictions, it could lead to comical misunderstandings among the enemy ranks, where they are so shocked or distracted that Naruto gets an advantage in battle.