Well, a major theme is the exploration of the unknown. Gaster's nature and his past are often shrouded in mystery in the original source, so in fanfiction, the reader and Gaster might embark on a journey to discover things together. Also, the theme of redemption can be present. Gaster may have some past mistakes or regrets, and the relationship with the reader could be a part of his path to redemption. Additionally, there can be a theme of power imbalance at times, as Gaster might have more knowledge or power, and how the reader deals with that in the relationship is explored.
A sense of otherworldliness is common too. Since Gaster Sans is from a rather strange and unique universe, the fanfictions often play with the idea of the reader being in a very different, almost alien world. This can lead to themes of adjustment, as the reader tries to fit into this new world while being in a relationship with Gaster Sans.
One common theme is mystery. Gaster is often a mysterious character in the original source, and fanfics usually keep that element. They explore how the reader tries to understand Gaster.
One common theme could be the exploration of their relationship in the context of the outertale universe. It might involve how they interact with the unique settings and other characters in that AU.
One common theme is mystery. Since both characters have elements of mystery in their original lore, fanfictions often explore that further. Another theme is power dynamics. Given their different abilities, how they interact in terms of power is often explored. And friendship or a deeper connection is also a common theme as writers like to explore the relationship between these two unique characters.
One common theme is mystery. Since both Gaster and Sans have mysterious backstories in the original game, fanfictions often explore and expand on these mysteries, especially in relation to their relationship.
One common theme is loyalty. They are both important figures in the monster world, and stories often explore how loyal they are to each other and to their respective causes.
A sense of belonging is another big theme. The reader, who may feel like an outsider in the real world, finds a place where they belong among the mutants. There are also themes related to fighting for justice, just like the X - Men do in the original stories.
Power and knowledge are also common themes. Gaster is often depicted as having a great deal of knowledge, and his experiments might be related to power in some way. Fanfics explore how this knowledge and power can be both a blessing and a curse. They might also show how other characters react to Gaster's power and knowledge, whether it's with fear, envy, or admiration. In addition, some fanfics have a theme of redemption. Since Gaster's actions may have caused some chaos or harm, the fanfic could be about him trying to make amends in some way, either directly or through the actions of others he has influenced.
One common theme is adventure. Since Chekov is part of the Star Trek universe, many fanfictions involve the reader going on adventures with him through space, exploring new planets and facing various challenges.
Love and sacrifice are also very common. The reader might fall in love with Vergil and be willing to sacrifice for him, or vice versa. In some fanfictions, there is also the theme of discovery. The reader discovers new sides of Vergil that no one else has seen, which deepens their relationship.