I'm not very familiar with 'drunk bunny x sans fanfic', but perhaps friendship could be a common theme. They might explore how these two characters interact in an unexpected or comical way.
The relationship could start as one of curiosity. Sans is curious about the drunk bunny's behavior. As time passes, it might turn into a friendship. Sans would be the more stable and reliable one, while the drunk bunny brings chaos and unpredictability. They could complement each other well.
I'm not sure about specific 'drunk bunny x sans fanfic'. You can try searching on fanfiction platforms like Archive of Our Own. They usually have a wide range of fan - created stories.
Maybe the drunk bunny is a lost soul in a strange dimension. Sans discovers the bunny and takes it under his wing. The fanfiction could then explore their relationship as the bunny sobers up gradually. For example, they could start to bond over shared experiences or interests. The bunny might also introduce Sans to some new things from its own world, creating an interesting cultural exchange within the story.
One common theme is friendship. Despite their differences from different AU's, they often form a unique bond.
One common theme is the struggle between good and evil, as Dream Sans is often associated with positive aspects and Nightmare Sans with negative ones. Their relationship in fanfics can explore how they deal with their opposing natures.
One common theme is redemption. Chara is often portrayed as a character in need of redemption, and Sans is sometimes the one who helps them on that path. Another theme is the exploration of different AUs, as it allows for a lot of creativity in how their relationship can develop.
A lot of these fanfics also deal with the theme of redemption. Nagito has his own complex backstory, and when paired with sans, there can be a story of redemption for Nagito. It can be about him finding a new purpose or a way to overcome his past through his relationship with sans. And then there's the theme of discovery. Discovering new things about each other, their powers, and their worldviews.
One common theme is the exploration of their different personalities coming together. Sans is usually depicted as cool and collected while Alphys is more nerdy and anxious. Their relationship often involves Alphys learning to be more confident with Sans' support. Another theme is the use of the Underground setting as a backdrop for their relationship to develop, like dealing with the other characters in that world.
One common theme is the exploration of good and evil. Flowey has done some rather cruel things, while Sans is often seen as a more 'good' character. Fanfics might play with the idea of Sans trying to redeem Flowey or vice versa.
One common theme is redemption. Often, Flowey, with his complex past, is on a journey towards redemption, and sans can play a role in that. Another theme is friendship in unlikely places, as they are two very different characters.