It could be that the baby is from a different species or culture. Hiccup is intrigued and wants to protect it. He may have to learn new things about this baby's origin. Maybe there are some who want to harm the baby, and Hiccup has to fight against them. The story could show Hiccup's growth as a character while dealing with this unexpected discovery of the baby.
He might be shocked at first. After all, finding a baby out of the blue is not an everyday occurrence.
The baby could be an alien species that Goku has never encountered before. It might have special powers related to its alien origin, like the ability to control energy in a different way than Goku and his friends.
The baby could be an ordinary human baby that has some sort of latent magical ability that Raven senses. She then decides to protect the baby from the human world that may not understand or accept the baby's potential magic, and also from the magical world that might want to exploit the baby.
The baby could be an orphan from a nearby village that has been attacked. Kagome stumbles upon the baby while on her travels with Inuyasha. Since she has a big heart, she can't just leave the baby there, so she takes it with her.
The baby could be a lost elf child from a different part of the forest. Maybe their family was attacked by some dark force and Thranduil finds them by chance.
The baby could be a half - demon, related to some other important character in the series. Maybe it's the offspring of a character Sesshomaru has some history with.
One possible plot could be that a magic spell gone wrong in Berk turns Hiccup into a baby. Toothless, being his loyal friend, tries to protect the baby Hiccup from the chaos that ensues. The other Vikings are confused at first but then start to take care of the baby Hiccup in their own clumsy ways, leading to some comical situations.
The plot could also center around the mystery of how Hiccup turned into a baby Night Fury. There could be an ancient magic or a curse involved. Hiccup, in his new form, starts to have strange dreams or visions that give him clues about how to turn back. Along the way, he discovers new things about the Night Furies that he never knew before, like some hidden abilities or a secret place that only Night Furies know. And he has to use these new - found knowledge and abilities to break the spell and return to his human form.
In the 'harry finds a baby fanfiction', there are numerous possibilities. It could be set in a world where Harry is a grown - up and he stumbles upon a baby that is crucial to the future of his world. The story might revolve around how he deals with this newfound situation. He could enlist the help of his friends to take care of the baby. There could also be a mystery surrounding the baby's origin that Harry is determined to solve, which could lead to all sorts of adventures and revelations.
I'm not sure specifically as I haven't read that particular 'lucy finds a baby fanfiction'. It could be about a character named Lucy who discovers a baby in a story that is fan - created. Maybe it's in a fictional world where she stumbles upon an unexpected baby and then various adventures or emotions unfold from there.