Well, a great plot for Catherine and Vincent game fanfiction could be them getting lost in a magical realm that is completely different from their own world. In this realm, they have to find a way back home while also dealing with the strange creatures and rules there. It could show how they rely on each other in a completely new and challenging situation, deepening their connection.
One idea could be that they face challenges in their relationship due to the pregnancy. Maybe their families have different reactions. For example, Vincent's family might be overjoyed but Catherine's family could be worried about the future. They could also struggle with preparing for the baby, like finding a safe place to live. Another aspect could be how they deal with the changes in Catherine's body and mood during the pregnancy.
There are those characters from the game who have special powers or unique personalities. For instance, that one character who has the ability to see the future in a hazy way. Including them can bring in an element of mystery. And then there are the characters who are on the fringes of Catherine and Vincent's world but have some knowledge about their relationship, like the old wise man in the alley who gives them cryptic advice from time to time.
One idea could be a fanfic that focuses on the backstory of the main character before the events of the game. For example, exploring his relationships with his friends or family that might have influenced his actions in the game.
One plot idea could be that Vincent and Cloud are on a quest to find a lost ancient relic. Along the way, they face various challenges and have to rely on each other's skills. Vincent's dark past and Cloud's internal struggles add depth to their journey.
There could also be fanfictions that are set during special events in 1987. For instance, if there were big cultural events or political happenings during that year, the story could incorporate how these events impact Vincent and Catherine. It could be something like they use a big cultural festival as a cover to meet in public or they have to deal with the after - effects of a political decision on their respective worlds.
The build - up of tension is important. If the story gradually shows their growing attraction and the obstacles in their way, like differences in their worlds or outside threats, then when they finally kiss, it's more impactful.
One plot idea could be that Vincent, before his death, made a secret pact with Undertaker that is only revealed years later. For example, it could be related to the future of the Phantomhive family and the underworld they are involved in. Another could be a story where Undertaker has memories of Vincent that he has never shared, and these memories start to surface when he meets Ciel, causing him to act in unexpected ways towards Ciel. Maybe there's a plot where Vincent's spirit is trapped in a limbo-like state, and only Undertaker can free him, but it comes at a great cost to the balance of the afterlife world in the Black Butler universe.
One idea could be that the Beast is initially overjoyed but then becomes overly protective, causing some tension between him and Catherine. He might start restricting her movements in the castle, thinking it's for her and the baby's safety.
In some fanfiction, it could be that making love is a way for them to heal each other's emotional wounds. Vincent, with his complex past, might find solace in Catherine's love. Catherine, on the other hand, could feel more complete. After this, they could decide to be more public about their relationship within their community, which would bring both support and some resistance, but they would face it together because of the new level of connection they have achieved.
One idea could be a wizard from a forgotten lineage emerging to challenge the established order in the wizarding world. Maybe he discovers an ancient magic that was thought to be lost.