In 'Steven x Pink Steven fanfiction', themes of identity are likely to be prominent. As they are two parts of a whole, the stories could revolve around how they define themselves both individually and in relation to each other. Also, themes of redemption could be present. Maybe Pink Steven has some past actions that need to be redeemed through his connection with Steven. And then there are themes of growth, as they both grow and change through their interactions.
I haven't come across a particular 'Steven x Pink Steven fanfiction' myself. But in general, fanfiction often takes the characters and their relationships in unique directions. Some stories might develop a deep friendship between them, while others could even create a more complex, interdimensional connection. You can start by looking for tags related to these characters on fanfiction platforms and see what catches your eye.
One possible theme is growth. Both Peridot and Steven grow throughout the original series, and the fanfic could continue that growth in their relationship. They might learn to understand each other better and become stronger individuals as a result.
A major theme is often power. As Pink Diamond, Steven has a different level of power and influence. Fanfictions might explore how he uses or misuses this power. Also, relationships are a big theme. His relationships with the Crystal Gems, his human friends, and even his enemies might change because of his new identity. For instance, his relationship with Garnet could be affected as she was made in part by Pink Diamond. There could be a lot of emotional turmoil and growth in these relationships.
Perhaps the fanfiction could start with Steven having a strange dream about Pink Diamond. When he wakes up, he starts to notice small changes in himself, like his gem glowing pink more often or having strange new abilities. As he becomes more and more like Pink Diamond, he realizes that there is a connection between his mother and himself that he never fully understood. He then embarks on a journey to discover the truth about his transformation, along the way facing challenges from both the Homeworld and Earth - based gems who are either afraid of or intrigued by his new form.
Adventure is also a common theme. They go on different quests or face various threats together. This could be something like exploring an ancient gem ruin on Earth or fighting off a new enemy. The theme of acceptance is important too. Accepting each other's flaws and differences. Amethyst accepting Steven's sometimes naive view of the world, and Steven accepting Amethyst's wilder side.
The theme of discovery is also prevalent. Steven discovers more about the Diamonds through his encounter with Pink Diamond, and Pink Diamond might discover new things about Earth and the Crystal Gems. There's also the theme of family, as Steven's connection to the Diamonds is a sort of family - related one, and this meeting can further explore that aspect.
One possible plotline could be that Lapis and Steven discover a hidden part of the Gem world together. They have to rely on each other's unique abilities to navigate through the challenges there. Another could be that Lapis is having trouble adjusting to life on Earth, and Steven helps her by showing her all the beautiful and positive aspects of the planet.
Perhaps a theme of acceptance. Pearl has her own past and way of dealing with things, and Steven is always so open - hearted. A kiss between them in fanfiction could symbolize Pearl accepting a new side of herself or a new type of relationship. Also, the theme of breaking norms as their relationship in the show is more of a friendship/mentorship, and fanfiction can play with changing that dynamic.
A theme could be cooperation. They have to work together to achieve certain goals, like protecting the Earth from threats. Additionally, the theme of self - discovery could be present. Through their relationship, they learn more about themselves. For example, Peridot might discover her own capacity for love and kindness that she didn't know she had before.
In addition to Steven and Pink Diamond, other characters from the Steven Universe series can also play important roles. For example, the Crystal Gems like Garnet, Amethyst, and Pearl might be involved. They could be skeptical or supportive of Steven's relationship with Pink Diamond, adding extra layers of drama to the fanfiction.