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?
Some fanfiction focuses on different relationships. For instance, exploring a friendship between Mustang and Hawkeye that develops in a different way than in the original story. It could show them going through challenges together that were not part of the main plot, like getting lost in an uncharted area during a mission and relying on each other to survive.
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 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.
One idea could be a crossover where Naruto's chakra system somehow gets intertwined with Alchemy. For example, Naruto could learn to transmute his chakra into different elements in a more alchemical way rather than just using ninjutsu. Another could be that characters from both universes meet in a strange in - between dimension. Maybe Sasuke and Edward Elric could have an intense battle or a deep conversation about their quests for power and revenge. And perhaps Hinata could learn some alchemical self - defense techniques from Winry Rockbell.
One idea could be a crossover where Harry Potter characters end up in the world of Fullmetal Alchemist. For example, Harry could use his magic in Amestris and face the challenges there. Another could be a story where the alchemy principles are somehow introduced into the Hogwarts curriculum. And a third idea might be a fanfic where a character from Fullmetal Alchemist attends Hogwarts and has to adapt to the magical world.
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.
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.
Maybe Edward dies in a fight against a powerful new enemy. This enemy could be someone who has mastered a dark form of alchemy that Edward couldn't counter. His death could then inspire the other characters, like Winry, to take up arms in a different way. She might use her skills as an automail engineer to create weapons that can defeat this new threat, all while mourning Edward and vowing to make his death count.
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.