One popular theme is redemption. Since Blue Diamond was initially an antagonist, many fanfictions explore her journey towards redemption through her interactions with Steven. Another theme could be friendship. Steven has a way of befriending almost everyone, and his relationship with Blue Diamond in fanfics can show a unique kind of friendship.
Exploration of their different powers is also a common theme. Steven has his gem powers and Blue Diamond has her own powerful abilities. Fanfictions might explore how they combine their powers or how they learn from each other in terms of using their powers. There are also fanfictions that focus on the emotional connection between them, like how Steven helps Blue Diamond deal with her past traumas.
Alternate universes are a really popular theme as well. In these fanfictions, the story might be set in a completely different universe where the rules of the Steven Universe world are changed. For example, Blue Diamond and Steven could be in a world where gems and humans live together peacefully from the start, and the fanfic could explore how they maintain this peace or face new challenges in this different setting.
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.
One common theme is redemption. Blue Diamond has done some not - so - great things in the past, and in the fanfiction, her relationship with Steven often serves as a path to her redemption. For example, Steven's kindness and understanding can make her see the error of her ways.
One common theme is the exploration of an unlikely relationship. Since Blue Diamond is such a powerful and complex character, it's interesting to see how she interacts with an ordinary reader - insert. Another theme is the redemption arc. Sometimes Blue Diamond is portrayed as having a change of heart through her relationship with the reader.
Maybe Steven tries to understand Blue Diamond's past and they form an unexpected friendship. For example, Steven might show Blue Diamond how the modern world has changed and how the gems are living differently now.
One great 'Steven and Blue Diamond fanfiction' is 'A Bond Beyond Worlds'. It delves into their unique relationship and how they interact in a world filled with magic and mystery. The story has a lot of heartwarming moments as Steven tries to understand Blue Diamond better.
There's also the 'secret relationship' trope. Considering their status in the Diamond Authority, a relationship between them might have to be kept a secret at first. Fanfictions might explore the challenges and thrills of keeping such a relationship hidden from the other Diamonds and the rest of the Gem society.
Both characters could develop a mutual respect for each other. Steven would respect Blue Diamond's long history and power, while Blue Diamond would come to respect Steven's unique position as a bridge between two worlds. This newfound respect could change how they interact with their respective groups as well.
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.
There's a fanfiction called 'The Meeting of Hearts' which is quite good. In this, Steven and Blue Diamond start to understand each other's perspectives in a more profound way. It shows how their interactions can lead to changes in the gem society as they both bring different values to the table. The writing is very descriptive, making it easy to picture the scenes in your head.
One idea could be that Steven tries to understand Blue Diamond's past traumas and helps her heal. Maybe he uses his unique powers of empathy to break through her cold exterior.