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 really interesting plot could be Scar crossing over into a fantasy world full of different races and magic systems. He could form an alliance with some of the oppressed groups in that world, much like he did with the Ishvalans in his own world. Or perhaps a crossover with a post - apocalyptic world where his skills in survival and combat are put to the test in a new and extreme environment.
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.
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 great 'Fullmetal Alchemist Scar Crossover Fanfic' is 'Scar's New Path'. It takes Scar on an adventure where he teams up with characters from another universe. The story is really engaging as it delves into Scar's character development in this new and strange setting.
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.
A good 'Fullmetal Alchemist Scar Crossover Fanfic' should first of all stay true to Scar's character. His backstory, his beliefs and his unique abilities should be respected. For example, if he's made to be too out - of - character, like suddenly being overly friendly without a proper reason, it won't feel right.
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?
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions, and you can search specifically for Fullmetal Alchemist Scar crossovers. Just use the search bar and type in relevant keywords. Another option is FanFiction.net, which also has a wide range of fan - created stories.
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.