One common theme is friendship. Many fanfics explore how Snow and Lucy become friends and how that friendship evolves over time. They might start as strangers or acquaintances and then develop a deep bond.
A lesser - known but really good Snow x Lucy fanfic is 'Lucy in Snow's World'. It shows how Lucy adapts to Snow's world, which is very different from her own. It has a unique take on their relationship as Lucy discovers new aspects of Snow and they grow closer through shared experiences.
Love and power struggle are common themes. Their relationship is often set against the backdrop of power dynamics, and the fanfics explore how their feelings for each other interact with their desires for power.
Common themes in 'lucy x sans fanfic' include trust - building. In these stories, Lucy and Sans usually start off not fully trusting each other. As the story progresses, they learn to trust one another more. Additionally, the theme of protection is often there. Sans might protect Lucy from various threats in their world, or vice versa, which helps to strengthen their bond and is a key part of many 'lucy x sans' fanfictions.
A common theme could be the exploration of their different powers. Since Lucy has magic from Fairy Tail and Ichigo has his soul reaper powers, fanfics might play around with how they interact with each other's abilities. For example, they could team up to fight a powerful enemy that requires both magic and soul reaper skills.
One common theme could be friendship development. Since they are from different series, exploring how they become friends in the fanfic world is interesting. Another might be adventure. Writers could create new adventures for them to go on together.
Since I haven't read this 'zefer x lucy mindbreak fanfic', it's hard to say exactly. However, generally, a 'fanfic' is a work of fiction written by a fan of a particular original work. Maybe Zefer and Lucy are from an existing series, and this fanfic focuses on breaking their normal mental states through various plot devices like a mysterious event, a powerful enemy, or a personal crisis.
Love is also a very prominent theme. Given that it's a 'naruto x lucy' fanfic, many authors explore the romantic aspect of their relationship. It could be a slow - burn romance where they initially don't get along but over time fall in love. Or it could be a more immediate attraction that they have to deal with while also facing the challenges in their respective worlds.