A common theme is redemption. Often, the reader comes into the Underfell world and tries to help the characters find a better path. Another theme is the power of love and friendship. Despite the often - dark and hostile environment, the connection between the reader and the characters can bring about positive change. Danger and survival are also themes, as the Underfell world is not a safe place.
Often, the cuteness of the baby form is a key feature. Despite being from the Underfell universe which is usually more edgy, the baby version brings out a different side.
One popular 'Overwatch Reacts' fanfiction could be the ones where the Overwatch characters react to different timelines or alternate universes. For example, they might react to a world where the Omnic Crisis was never resolved. Another could be their reactions to new heroes joining the team. It's all about exploring the characters' personalities through these reaction scenarios.
Well, usually they focus a lot on the Doctor's reactions and emotions to various situations. The fanfictions might also explore how his character develops in unexpected ways.
One popular 'baby underfell sans fanfiction' is 'The Tiny Terror'. It has an interesting take on the character in a baby form, with unique plot twists and character interactions.
They might be surprised by the complex and often dark themes in Warhammer fanfiction. RWBY characters are used to their own universe's rules and aesthetics, so the gritty and elaborate world of Warhammer could be a shock.
In the 'hogwarts reacts to muggle stuff fanfic', the students and teachers might react in different ways. The younger students could be overly excited. They might see a muggle mobile phone and think it's some kind of magical communication device that muggles have invented without magic. They would be eager to play with it. However, the older students and teachers might be more cautious. They might be a bit worried about how these muggle things could potentially affect the wizarding world.