Often, these fanfictions blend the dark and complex world of Fullmetal Alchemist with the idea of 'lust' which could be in terms of a character's desires or a new, alluring force in the story. They might also explore different character relationships and how 'lust' can either strengthen or disrupt them.
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.
Some fma crossovers involve characters from DC Comics. Picture Batman or Superman in the world of FMA, dealing with the concept of equivalent exchange and the political intrigue in that world. It can lead to some really unique storylines and character interactions.
One great fma scar crossover fanfiction could be where Scar teams up with a character from another universe to fight a common enemy. For example, a crossover with a character from Naruto. Scar's unique alchemy skills combined with the ninja's jutsu would make for an exciting plot.
A lesser - known but very creative fma envy crossover fanfiction is the one that combines the world of FMA with a steampunk - themed universe. Envy has to adapt to the new rules and technology in this world, and the story does an excellent job of showing this transformation.
One idea could be that Naruto characters are somehow transported to the FMA world and have to adapt to the rules of alchemy. For example, Naruto might be intrigued by the concept of equivalent exchange and try to apply his ninja way to it. He could learn alchemy and use his chakra in unique ways to manipulate it.
First, know both universes well. For an fma crossover, you need to understand the rules of alchemy in FMA and the characteristics of the other universe you're crossing it with. For example, if it's a Star Wars crossover, know about the Force and lightsabers as well as alchemy.
To write a good fma crossover fanfic, start by choosing the right crossover. It should be something that has potential for interesting interactions. For instance, if it's an FMA and Bleach crossover, think about how alchemy and soul reaper powers could interact. Research both fandoms thoroughly. Look at their cultures, technologies, and power systems. When writing the story, focus on the emotions of the characters. Show how they react to this new and strange crossover world. Use vivid descriptions to bring both universes to life in the fanfic. And don't forget to proofread and get feedback from other fans.
First, know the characters well. Understand Greed's personality, his desires, and his powers in FMA. Then, research the other fandom you want to crossover with. Next, create a unique and engaging plot that combines elements from both worlds in a seamless way. For example, if it's a crossover with a fantasy world, think about how Greed's alchemical nature would interact with magic. Don't forget to develop good character relationships and let the story flow naturally.
One idea could be that Greed finds himself in an alternate universe where the rules of alchemy are slightly different. He could then try to adapt and use this new knowledge to his advantage, all while facing new and unexpected enemies. Another plot could involve Greed teaming up with characters from another fandom in a quest for a powerful and mysterious object that exists in both worlds.