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 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.
Their relationship could develop through mutual respect. If they are constantly thrown into situations where they have to rely on each other's skills. For example, in a battle against a powerful alchemist, Lust's combat abilities and Edward's alchemical knowledge could complement each other, leading to a grudging respect that might grow into a friendship of sorts.
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 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 fanfiction could be where Edward and Alphonse travel to a parallel world. In this world, they have different roles and challenges to face. It shows a different side of their characters as they adapt to this new environment.
One possible fanfiction could be about Edward and Roy teaming up for a dangerous mission. Maybe they have to infiltrate an enemy stronghold where they discover a new type of alchemy that they've never seen before. Along the way, they start to understand each other better and respect the other's skills.
One idea could be a story where Edward and the reader are newly married and they have to face challenges from the alchemy world. Maybe some old enemies resurface and they have to use their combined skills to protect their new family. Another plot could be about them starting a new life in a small town, away from the chaos of alchemy, but still getting involved in local mysteries that have an alchemical twist.
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.