It could be another gem who doesn't want Steven and White Diamond to form an alliance. Maybe a rogue gem who fears that their combined power would disrupt the current power balance in the gem universe.
Perhaps the main antagonist could be an old enemy of White Diamond who sees Steven as a tool to get to her. This enemy might try to manipulate Steven into doing something that would harm White Diamond, all while hiding their true intentions from him. For example, they could pretend to be a friend to Steven and feed him false information about White Diamond's past deeds to make him turn against her.
There's a possibility that the main antagonist is a part of Steven himself. His own doubts and fears could be personified in the story. For instance, he might be afraid of getting too close to White Diamond because of her past actions. This internal struggle could act as an antagonist, preventing him from fully connecting with her and achieving a positive outcome in their encounter.
Well, perhaps in the fanfiction, Steven meets White Diamond when he's on a quest to understand more about the Diamonds' past. White Diamond is initially cold to him, but Steven's kind heart and unique perspective start to break through her icy exterior. They could end up having a deep conversation about forgiveness and change, which might lead to a new alliance or at least a better understanding between them.
Another option is an ancient evil that was once sealed away by the Diamonds long ago. But due to some recent events in the Steven Universe universe, it is starting to break free. This evil force could target Blue Diamond because of her past actions and Steven because of his potential to stop it.
Steven, the half - human, half - Gem protagonist, and Yellow Diamond, a high - ranking Gem. In this fanfiction, they are at the center of the action. Steven's character is full of compassion and a desire for harmony, while Yellow Diamond is a complex character with her own set of values and power - driven mindset.
It could be a corrupted Gem from the past that was somehow locked away and now resurfaces in the future, angry at the changes that have occurred. This Gem might not understand the new world order and wants to return things to how they were.
Perhaps Steven and Yellow Diamond end up having a long - awaited conversation. They could discuss the past, the reasons for the war between the Gems, and Steven could share his unique experiences as a half - human, half - Gem. This might cause Yellow Diamond to question her long - held beliefs and start to consider a different path for the Gem Homeworld.
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.
Obviously, Steven and Pink Diamond are the main characters. Steven, with his unique ability to understand and empathize with all beings, and Pink Diamond, with her complex and mysterious past. Their interaction would be the core of the fanfiction.
One possible plot could be that Steven, having heard so much about Pink Diamond, finally has a chance encounter with her in a dream-like realm. They start to talk about the past, how Pink Diamond used to view the Earth and her complex relationship with the other Diamonds. Steven tries to understand her better and in the process, Pink Diamond gets to see how much the Earth has changed and how her actions still have an impact today.
A powerful rogue gem who has been stealing energy from other gems and growing stronger could be the main antagonist. This gem could be jealous of Blue Diamond and Yellow Diamond's power and status and aims to take them down.
A group of Homeworld Gems who believe that Black Pearl has become too 'contaminated' by her interactions with the Crystal Gems and Steven. They see her as a threat to the traditional Gem order and send a special task force to capture or destroy her, which would make them the main antagonists as they try to stop Black Pearl from living her new life.