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.
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 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 really interesting fma crossover fanfic idea might be with the world of Fullmetal Alchemist and the Lord of the Rings. Imagine the Elric brothers in Middle - earth, facing the challenges there. They could use their alchemy skills in a world filled with magic, elves, and orcs. It would be a very unique combination as the concepts of alchemy and the magic in Middle - earth are quite different.
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.
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.
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.
One common theme could be the clash of different magical systems. In Harry Potter, there's the magic based on spells and potions, while in FMA there's alchemy. Another theme might be the exploration of different cultures and values. The pride aspect could also play into characters showing their unique identities and abilities from both universes.
Check out some FMA fanfiction review blogs as well. These blogs often feature and recommend different types of fanfictions, including the crossover ones with Envy. They might give you a good idea of which ones are worth reading.
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.