One likely character could be a rival alchemist who is jealous of Edward's skills and reputation. Another might be a villain from the original story who has a grudge against Edward. Also, a new character created specifically for the fanfiction, like a power - hungry alchemist who thinks Edward can help him achieve his goals.
Well, it could be a rogue military officer who wants to use Edward's alchemy for his own personal gain. He might see Edward as a tool to get ahead in the military hierarchy. Or perhaps a mad scientist - type character who is obsessed with the idea of unlocking the full potential of alchemy through Edward. There's also a chance it could be a character from another dimension or world who kidnaps Edward because they believe he holds the key to something important in their own realm.
There's a chance that Scar could betray Edward in certain fanfiction concepts. Scar has his own motives and a complex past. If he feels that Edward is getting in the way of his revenge or his understanding of alchemy, he might betray him. Additionally, some fanfictions might explore the idea of an old acquaintance from Edward's childhood betraying him. Maybe someone who was once a friend but has since been corrupted by power or a dark influence. And then there's always the possibility of a new, mysterious character being introduced specifically to betray Edward for some yet - to - be - revealed reason.
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.
Obviously, Edward Elric himself is a very popular character. He's the protagonist of the original story and his return is the central theme, so he's always in the spotlight in these fanfictions.
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.
Batman's allies are very likely. For example, Robin. The Joker would see kidnapping Robin as a great way to taunt Batman. Also, Commissioner Gordon might be a target as he represents law and order, which the Joker loves to disrupt.
The main characters would be Hermione, the victim of the kidnapping. The Lestrange brothers are the antagonists. Neville Longbottom could also be involved. He might have some unexpected role, like using his newfound courage and knowledge of plants to help in the rescue mission.
Obviously, Edward Elric himself is the central character in this type of fanfiction. His brother Alphonse might also be a common character, often trying to help Edward deal with his new situation. Winry Rockbell could be there too, perhaps having a different kind of relationship with Edward now that he is a girl. There might also be the appearance of some of the villains from the original story, who could either take advantage of Edward's new form or be involved in the reason why he turned into a girl in the first place.
Winry Rockbell is often paired with Edward. Their relationship from the original series can be further explored in the alternate world setting. Maybe in this new world, they have different challenges in their relationship, like different social statuses or new enemies that target their relationship.
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.
I'm not sure who the absolute best is, but there are some authors who are well - known in the fanfiction community. Some of the more popular ones might be those who have a deep understanding of the 'Fullmetal Alchemist' universe. They can really capture the essence of Edward and Hohenheim's characters and their relationship.