A plot could involve a crossover where Pride somehow gets trapped in a human body like the main characters in Fullmetal Alchemist. He has to adapt to human emotions and limitations while also dealing with the alchemists who are initially very suspicious of him. This would be a great opportunity to explore Pride's character in a different light and see how he copes with his new situation.
Perhaps in the fanfiction, Pride's powers could be combined with alchemical knowledge. For example, the alchemists could try to use Pride's unique abilities in their own transmutations. This would create a whole new form of alchemy that has never been seen before in the Fullmetal Alchemist universe. It could also lead to a power struggle as some characters might want to use this new form of alchemy for their own gain.
One plot idea could be Scar crossing over into a world where alchemy doesn't exist, and he has to adapt to a completely different form of combat and magic. Another is a crossover with a modern - day world, and Scar has to deal with modern technology and society while still holding onto his beliefs.
One idea could be to have Bradley cross over into a modern - day world. He'd be out of his element, dealing with things like technology and modern social norms. It could be funny to see his reaction to things like cell phones or cars.
One idea could be that Naruto characters somehow end up in the world of Fullmetal Alchemist. Maybe Naruto and Sasuke are chasing after a rogue ninja who uses some strange alchemy - like jutsu to open a portal to the other world. There, they have to learn about alchemy to find their way back home and in the process, they meet the Elric brothers and team up to fight against common enemies, like homunculi who are interested in the power of chakra.
One idea could be the alchemist using gamer skills to unlock new alchemical formulas faster. For example, they might complete certain quests in a virtual - like world within the fanfiction to gain knowledge of rare alchemical combinations.
Another idea might be alternative endings. Fanfiction often plays with the concept of 'what if' things had gone differently. Maybe if Edward had not been able to regain his brother's body, how would he have coped and what new path would his life take?
One interesting concept could be the combination of different alchemical abilities from various characters in different universes. For example, if a character from another anime with a unique power system crosses over into the Fullmetal Alchemist world, it could lead to new alchemical reactions and techniques. Another concept might be the exploration of how different cultures and moral codes interact. Characters from different fictional worlds might have different views on equivalent exchange, which could create some really thought - provoking scenarios. Also, the idea of crossovers can introduce new relationships between characters. Characters who have never met before might form alliances or rivalries based on their own goals and personalities.
One idea could be exploring Lust's backstory before she became a Homunculus. Maybe she had a human life before and what led to her transformation. Another could be a fanfiction where Lust forms an unlikely alliance with a human alchemist. They could work together to achieve a common goal, like saving a town from a greater evil.
One idea could be an OC who is a foreign alchemist with a unique form of alchemy based on their homeland's culture. For example, an alchemist from a desert region who uses sand-based alchemy. Another could be an OC that has a special connection to the Homunculi, not as an enemy but as someone trying to understand and redeem them. Maybe an OC that was experimented on by the same people who created the Homunculi and has some shared traits.
One interesting plot idea could be that Edward and Lust are forced to team up against a common enemy. Maybe a new alchemic experiment gone wrong has created a powerful and dangerous being that threatens the world they know. Their combined knowledge of alchemy and Lust's unique abilities might make for an interesting dynamic as they struggle to defeat this foe.
One idea could be a story where Ed and Al find a way to reverse their alchemical mistakes in a completely unexpected way. Maybe they encounter a long - lost alchemist who has a unique method. Another plot could be them traveling to a parallel universe where their roles are reversed. For example, Al is the one who is more assertive and Ed is more reserved.