One common theme is the growth of their relationship. As they work together in the ANBU, they get to know each other better.
One common theme is secrecy. Since they are in the Anbu, their relationship has to be hidden from others. Another theme is the growth of both characters. Naruto is constantly evolving, and Yugao also has her own character arc. And of course, there's the theme of loyalty, both to the village and to each other within the context of their relationship.
One common theme is the exploration of their relationship in the context of the ninja world. For example, how they balance their duties as ninjas with their growing feelings for each other.
One common theme is growth. Naruto is often shown growing as a ninja with the influence of Yugao Uzuki. Another theme is friendship. They start as comrades and the fanfiction may explore how that friendship deepens.
Common themes include adventure and discovery. As they are in the Naruto universe, they might go on missions together and during that process, their relationship blossoms. There's also the theme of secrecy. In some fanfictions, their relationship has to be kept a secret for various reasons, which adds an element of mystery and tension. Additionally, the theme of sacrifice often comes up, where one or both of them might be willing to sacrifice something for the other's sake.
The theme of discovery is also prevalent. They discover new things about each other, about the ninja world, and about themselves. In some fanfics, they might discover hidden powers or secrets within Konoha while working together.
One common theme is the development of their relationship against the backdrop of the dangerous ANBU world. Another theme is their growth as individuals within the ANBU.
A prevalent theme is sacrifice. Given the dangerous nature of Anbu missions, there are often situations where one of them has to make a sacrifice for the other or for the greater good of the village. This theme adds a lot of depth and drama to the fanfictions. It also shows the strength of their relationship as they are willing to put themselves in harm's way for each other.
The theme of protection is very common. Naruto, as an Anbu, is highly skilled and uses his abilities to safeguard Sakura. There's also the theme of unrequited love at times. In some fanfictions, Sakura may not be aware of Naruto's feelings at first because of his secret Anbu life, and Naruto has to find a way to make her understand his love.
For Naruto in Anbu, you could start with his initial brashness still showing but slowly being tempered by the strict Anbu code. As he interacts with Yugao, he starts to see things from a different perspective. Yugao, on the other hand, could start as a reserved and focused ninja. When she meets Naruto, his energy starts to break down her walls. You can show their development through their conversations and how they react to different situations together.
In fanfiction, Naruto and Yugao could have a mentor - mentee relationship. Yugao being an Anbu has a lot of skills and knowledge that she could pass on to Naruto. It could be interesting to see how Naruto, with his energetic and determined nature, would respond to her more stealthy and mysterious Anbu ways.