One common theme could be obsession. Since Billy is a yandere, there would likely be an extreme and often unhealthy obsession with Spencer.
I'm not sure of specific 'yandere billy x spencer fanfic' titles off - hand. However, you could also check out some fanfic communities on Reddit. Sometimes, fans share and discuss their favorite fics there, which might lead you to the 'yandere billy x spencer' ones you're looking for.
A common theme could be the exploration of their different personalities. Spencer Reid is often portrayed as highly intelligent and a bit socially awkward, while Eliot Spencer is more of a tough, action - oriented type. So, fanfics might focus on how they interact and learn from each other.
Growth and self - discovery are important themes too. Fanfics might show Aria and Spencer growing as individuals, learning more about themselves as they face different situations, whether it's about their careers, relationships or personal beliefs. For instance, they could be struggling with choosing a major in college and through their conversations and experiences, they find their true passions.
Friendship is a very common theme. Aria and Spencer's deep friendship is often explored, like how they are always there for each other. Another theme is mystery, as they are often involved in solving some sort of mystery in the fanfics.
One common theme is over - protectiveness. The yandere character, being pregnant, may become extremely possessive and protective of their unborn child and their partner. For example, they might not allow anyone to get too close to them or the baby.
One common theme is possessiveness. Yandere girls in these fanfics are often overly possessive of the person they love. They don't like to share and will do anything to keep their loved one away from others.
One common theme is possessiveness. Albedo, in these fanfics, often wants to possess the person he has feelings for completely. For example, he might not allow them to interact with others freely.
In yandere Hermione fanfic, the theme of transformation is quite common. Hermione, who is typically known for her intelligence and level - headedness, undergoes a transformation into a more unhinged and obsessive character. Another theme is the power dynamics. Her yandere nature can often create a power imbalance in her relationships. For instance, she may use her intelligence in a devious way to manipulate situations to her advantage in relation to the person she loves. Additionally, the theme of self - destruction sometimes emerges. As her yandere tendencies progress, she may start to harm herself or her relationships in the long run due to her extreme actions and emotions.
Isolation is also a common theme. Yandere characters in these fanfics often try to isolate the BTS member from their friends, family, or other fans. They want the member all to themselves and will go to great lengths to achieve this, which creates a lot of drama and conflict in the stories.
One common theme is possessiveness. Yandere characters in RWBY fanfic often display an extreme need to possess their love interest. Another theme is the distortion of love. Instead of healthy love, it's a warped version where the yandere will do anything to keep their beloved, even if it means harming others. Jealousy is also a big theme. They can't stand anyone getting close to the object of their affection.