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.
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.
Well, it depends on what you're looking for in a fanfiction. Some 'Steven is Pink Diamond' fanfictions are more action - packed, with Steven using his new - found power and status to fight against new threats. Others are more character - study - like, delving into his internal struggles and how he copes with being Pink Diamond. You really have to dig in and read a few to find the ones that suit your taste.
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.
Power and responsibility is also a theme. Given that Steven has unique powers and Blue Diamond has her own vast power and influence. Fanfictions often explore how they use or should use their powers and the responsibilities that come with them. In some stories, they work together to solve problems that require both of their powers and the right mindset to take on the associated responsibilities.
In 'pink diamond time travel fanfiction', a recurring theme is the mystery surrounding the diamond's time - traveling ability. Writers often explore how and why the pink diamond has this power. Another theme is the interaction between the diamond and different cultures throughout time. For example, how it is perceived in ancient civilizations compared to modern ones. There is also the theme of love and friendship that develops as characters travel through time with the pink diamond. The shared experiences create strong bonds between them.
Love and loyalty are common themes. Since Pearl was so devoted to Pink Diamond, many fanfictions explore the depth of that loyalty in different ways, like how it endures even after Pink Diamond's 'disappearance'.
In 'pink diamond x yellow diamond fanfic', a common theme is the concept of change. How they change over time, either due to external factors or because of their relationship with each other. For example, yellow diamond might become more empathetic because of pink diamond, or vice versa. Another theme is the idea of legacy. Since they are diamonds, their actions and relationships can have a significant impact on their legacy, and this is often explored in fanfics.
Well, I'm not sure of a specific 'Steven becomes Pink Diamond' fanfiction off the top of my head. However, you could also try looking on FanFiction.net. There are so many fanfics there, and if you use the right search terms, you might come across an interesting one. Some might explore how Steven's character changes when he takes on the role of Pink Diamond, while others could focus on the reactions of the other characters in the story.
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.
One interesting idea could be the cultural exchange between Pink Diamond and Steven. Since they come from different backgrounds within the Gem world, their meeting might lead to a sharing of unique abilities and knowledge.