A third theme could be growth. They both start from certain points in their characters and develop over the course of the fanfic. Tobi might become more serious or Zetsu might open up more, depending on the plot and how the author wants to develop their relationship.
One main theme is loyalty. Given their relationship in the Naruto series, fanfictions often explore how loyal they are to each other and their common goals.
I'm not sure specifically about 'Tobi x Zetsu' yaoi fanfiction. You could try searching on popular fanfiction websites like Archive of Our Own or FanFiction.net. They usually have a wide variety of fanfictions, and you might be able to find some related to this pairing there.
One popular tobi x zetsu fanfiction is 'The Bond of Shadows'. It delves into their unique relationship and the adventures they might have together in the Naruto universe.
There's also 'Tobi and Zetsu: A Different Perspective'. This fanfic delves deep into their backstories and how those past experiences might influence their relationship. It's really interesting as it gives a fresh take on their characters and how they could have come to be the way they are in the Naruto universe.
FanFiction.net could also be a source. Just be aware that the tagging system might not be as detailed as AO3. You'll need to dig a bit deeper by using different search terms related to the characters and the yaoi genre. Additionally, some fan - made forums dedicated to the relevant fandom might have links or discussions about such fanfictions.
The theme of discovery is also prevalent. Kagome discovers new aspects of the Naruto world through Zetsu, and Zetsu discovers new emotions and ways of thinking because of Kagome. This mutual discovery creates a dynamic that drives many of the fanfictions in this pairing.
The Tobi x Deidara comics often feature intense battles and strategic showdowns between the two characters. They also explore their complex relationship and mutual interactions.
The setting also plays a big role. Whether it's in the original Naruto universe or an alternate one. In some 'tobi x naruto fanfic', the authors create new locations or change the existing ones to fit their story. And of course, the character backstories are important. Tobi's mysterious past and Naruto's journey to become Hokage are often used to add depth to the plot.
A very common theme is the clash of personalities. Tobi is often depicted as this somewhat unpredictable and shadowy figure, while Sakura is strong - willed and determined. So, their interactions are full of tension. Also, the theme of trust - building is quite prevalent. In many fanfics, Sakura has to learn to trust Tobi despite his mysterious nature.