You should begin with a clear idea of what makes your Undertale AU different. Develop interesting character backstories and interactions. And make sure the dialogue is engaging and fits the tone of the comic.
Well, they might see different versions of characters from Undertale. For example, the RWBY characters could be really surprised by the unique battle mechanics in Undertale's AU multiverse. Maybe Ruby would be especially interested in the different types of weapons or magic used in that world.
They might be really intrigued. Ruby, with her love for adventure stories, would probably be excited to see the unique worlds and characters in the fanfiction. Weiss might be a bit more critical at first, analyzing the writing style and how it adheres to the original Undertale lore. Blake would likely be interested in the deeper themes and character relationships presented in the fanfiction. Yang, on the other hand, would enjoy the action parts and cheer on the characters during intense scenes.
Blake could also be a great character to focus on. Given her own complex backstory and her tendency to be more introspective, she might have deeper insights into the characters and situations in the Undertale AUs. She could form unique relationships with some of the more morally ambiguous characters in the Undertale multiverse.
If it's a comedy - based fanfiction, the character might be laughing out loud or at least have a big smile on their face. They'd be enjoying the humorous take on their world. For a more serious, angsty fanfiction, they could be feeling solemn or even a bit sad, especially if it involves difficult situations for their friends or themselves.
One main theme is friendship. Just like in the game, many short stories focus on the relationships between the characters, such as the unlikely friendship between a human and a monster. Another theme is redemption. Some stories might show how a character who has made mistakes tries to make up for them.
Well, mystery is a big one. Given that 39 clues is centered around mystery, it's often incorporated into the Undertale setting in these fanfictions. Also, friendship and cooperation. Just like in the original works, the characters in the fanfiction might team up to solve problems or face challenges together. And sometimes, there's a theme of self - discovery as the characters navigate through the combined worlds of 39 clues and Undertale.
One common theme is probably the exploration of relationships between male characters in the Undertale world. It could be about how they support each other and grow closer.
Another theme is the struggle between good and evil. The ocs can be faced with moral choices, whether to side with the more 'good' characters in Undertale or be tempted by the 'evil' forces. This adds depth to the story and makes it more interesting.
One common theme is the change in relationships. For example, how the character's partner or friends react to the pregnancy. Another theme is the anticipation and fear of the unknown. Since the Undertale world has its own unique elements, the pregnancy might bring new challenges and uncertainties. Also, the theme of protection often comes up, with the character and those around them being protective of the unborn.