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 '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.
Other monsters from the Undertale world can also be main characters. For instance, Toriel might be a mother - like figure to Baby Bones in some fanfictions. She could offer him guidance and protection, adding a different dynamic to the story.
The theme of family values also pops up a lot. The fanfictions might show how their families (both Booth's and Bones' extended families) react to the new baby, and how they all come together to support the new little family. It's about the importance of family and the new family unit that Booth and Bones create with their baby.
In such a fanfiction, their relationship would change in many ways. They would have to communicate even more effectively as they make decisions about the baby's future. There could be some power struggles too, like Booth wanting to be very traditional in some parenting aspects while Bones has her own scientific views. But overall, they would grow closer as they face the challenges and joys of impending parenthood together. They would also start to see each other in a new light, not just as partners at work but as co - parents with a shared responsibility for a new life.
There could be fanfictions that explore the baby's early development and how it impacts the relationship between Bones and Booth. Maybe the baby has some unique qualities that add an interesting twist to their lives. Some stories might also include the reactions of their colleagues at the Jeffersonian to the new addition to the family.
Fanfiction.net is also a popular platform. Just search for 'Bones Booth Baby' in the search bar, and it should show you the relevant stories. However, the quality and content can vary widely, so you might need to sift through a bit.
One interesting undertale baby bones fanfiction could be about his growth and development in the underground world. Maybe it shows how he learns to use his powers at a young age.
There are quite a few out there. 'Baby Bones' Friendship Quest' is a great one. It focuses on Baby Bones trying to make friends in the rather strange world of Undertale. The author does a wonderful job of showing Baby Bones' innocent nature and the challenges he faces while trying to form connections with other characters.