Well, the theme of cooperation is quite common. They come from different worlds (Ichigo from the human world and Rukia and Renji from the Soul Society) and yet they have to cooperate to face threats. Also, the theme of sacrifice is there. In many fanfictions, one of them might be put in a situation where they have to sacrifice something for the others. And of course, the theme of love, not necessarily romantic love all the time, but love in the sense of caring deeply for each other and their shared values.
One common theme is friendship. Their bond as comrades is often explored, like how they rely on each other in battles. For example, in many fanfictions, Renji is there to back up Ichigo when he's facing a powerful enemy.
A popular 'ichigo x rukia x renji fanfiction' is 'Soul Connections'. It starts with a strange phenomenon in the Soul Society that links the destinies of Ichigo, Rukia, and Renji. As the story progresses, we see how their pasts, especially Rukia and Renji's history together in the Gotei 13, influence their present relationships with Ichigo. The fanfiction does an excellent job of portraying the growth of their relationships, from initial misunderstandings to a deep and unbreakable bond.
The theme of self - discovery also appears frequently. In these fanfictions, Ichigo and Renji may discover new aspects of themselves through their relationship. For example, they might learn to be more vulnerable or understanding. And sometimes, the theme of sacrifice is there, where one character is willing to sacrifice for the other in the name of love.
One popular theme is adventure. Writers often create new adventures for Rukia and Ichigo, where they face powerful enemies together and their relationship deepens through these challenges.
In many 'ichigo x renji fanfic', the theme of partnership is quite prevalent. They are often portrayed as a strong duo, relying on each other's strengths. The theme of acceptance is also there. They accept each other for who they are, with all their flaws. Additionally, the exploration of their cultural differences (as they come from different backgrounds in the story) can be a unique theme in some fanfics.
The theme of friendship - turned - love is often explored. They start as friends or comrades in the Soul Society, and then their relationship blossoms into something more. There's also the theme of overcoming obstacles. These obstacles can be external, like the rules of the Soul Society, or internal, like their own insecurities. For instance, Renji may struggle with feeling worthy of Rukia's love, and the fic will be about how he overcomes that self - doubt while they build their relationship.
A prevalent theme in such fanfiction is the power of love to overcome differences. Ichigo is human (sort of), and Rukia is a Soul Reaper. Their love defies these boundaries. Also, the theme of loyalty often shows up, as they are loyal to each other through thick and thin.
Another theme could be power - up or training arcs. Since both Ichigo and Rukia are involved in battles and have powers, fanfictions sometimes focus on them training together or unlocking new powers while being in a relationship. This not only shows their growth as individuals but also how they support each other in their journey to become stronger.
One common theme is friendship. Their long - standing friendship is often explored, showing how they've been there for each other through different challenges in the Soul Society. Another theme is loyalty. Renji's loyalty to Rukia and vice versa is a central aspect in many fanfictions. It might be shown through how they protect each other during battles or when facing political intrigue in the Soul Reaper world.
Another common theme is the 'what - if' scenarios. For instance, what if Renji had made different choices in certain situations regarding Rukia? This allows for a lot of creative exploration in the fanfictions. And also, stories about them having to work together against a common enemy while also dealing with their unspoken feelings for each other are quite popular.