Another theme could be innocence. Baby Edward is in a pure state, and fanfictions might explore how this innocence is gradually lost as he grows up and faces the hardships of the alchemical world. For example, a story could show how his first encounter with the darker side of alchemy starts to chip away at his innocent view of the world. It could be something as simple as seeing an alchemical experiment go wrong and realizing that there are consequences to the power he's so interested in.
Yes, there are likely fanfictions out there with such a concept. The worlds of Naruto and Fullmetal Alchemist, where Edward Elric comes from, are quite different, but that's what makes the idea of a crossover in fanfiction so interesting. Writers might explore how Naruto's ninja world and its unique powers could interact with Edward's alchemical abilities and the complex world of Amestris.
One common theme is innocence. Since it's about baby Edward, his innocence is often explored. How he views the world without the full understanding of the harshness that will come later.
One interesting plot could be where Edward has to adjust to sudden fatherhood while still dealing with his alchemical pursuits. Maybe he discovers a new form of alchemy related to protecting his child.
There could also be some original characters created just for the fanfiction. Maybe a new female alchemist who crosses paths with Edward and they end up having a relationship that leads to him becoming a baby daddy. Another possibility is an old acquaintance from his travels who resurfaces and they develop feelings for each other, resulting in a child.
Alphonse Elric is often present. He might play a role in helping Edward understand and use his new gamer - like abilities in relation to alchemy. For example, he could be the one who discovers a new aspect of Edward's powers.
Edward might meet his parents. Since he has the chance to go back in time, he could meet them when they were alive and perhaps learn more about alchemy from them, which could change his future actions.
One possible plot could be that his friends have to take care of him while trying to find a way to reverse the transformation. They might struggle with things like feeding him, changing his diapers, all while protecting him from enemies who could take advantage of his vulnerable state.
Alphonse would be very protective. He'd probably be constantly worried about his baby brother's safety. He might be a bit clumsy at first with taking care of a baby, but his determination would make him learn quickly.
One popular 'baby Edward Elric fanfiction' might be those that explore his early days with his brother Alphonse. Some stories focus on how their bond started to form even at a young age. For example, there could be a story where baby Edward shows early signs of his intelligence and determination, which later become his defining traits.