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 might involve Goku finding the baby as part of some sort of mystery. For example, the baby could be a key to unlocking a hidden power or an important prophecy. Goku, being the hero he is, decides to protect the baby at all costs. He may seek help from his friends like Bulma to figure out more about the baby and its significance.
Well, it could be a heart - warming plot where Sakura discovers an abandoned baby and decides to take care of it. Maybe she has to face challenges like hiding the baby from authorities at first because she doesn't trust them to do right by the child. And then as the story progresses, she might find help from unexpected friends who support her in this new role of a surrogate mother.
Well, it could be that Dean stumbles upon a baby in some unexpected place like an abandoned building. He then has to figure out what to do with the baby, whether to find its parents or take care of it himself for a while.