One common theme is identity. Since the homunculi are in a new crossover world, they often question who they are and their place in this new setting. For example, in many crossovers, they might compare themselves to the new characters they meet.
There's also 'Homunculus in Another Realm'. In this fanfic, the homunculi are sent to a parallel universe. It has an interesting plot where they interact with new characters and face different challenges than in the original FMA world. The author does a great job of portraying their personalities in this new setting.
A great part of Homunculus Edward fanfiction is the creativity in the plot twists. Writers can introduce unexpected elements. Like, what if Edward's alchemy was affected by a homunculus in a way that made him able to do things he never thought possible? This can lead to really engaging stories that keep readers on their toes.
To create a compelling OC homunculus, start with its motivation. What does it want? Is it power, acceptance, or something else? Then, think about its relationships. Does it have an ally among the human alchemists or is it at odds with all of them? Also, consider its powers. Make them interesting and related to the Fullmetal Alchemist lore. For instance, it could have the ability to manipulate a specific type of metal in a way that's different from the main characters. And don't forget to give it flaws. A homunculus that is too perfect won't be as engaging.
I'm not sure specifically as there could be many interpretations. It might be a fan - created story where a character named Percy is depicted as a homunculus. A homunculus in some fictional concepts is a small, artificially created human - like being, so perhaps the fanfiction explores Percy's adventures or experiences as such a being.
A 'homunculus' in fanfic often refers to a being created through artificial means. In the context of 'harry is a homunculus fanfic', it might be about Harry being transformed into or discovered to be a homunculus. It could explore how this changes his relationships, his powers, and his place in the wizarding world. For example, if he's a homunculus, his magic might be different, or his connection to other characters could be altered. Maybe he has to deal with the stigma associated with being an artificial creation within the magical society.