One idea could be to have the characters from 'Fullmetal Alchemist' (FMA) end up in the 'Ouran High School Host Club' (OHSHC) universe. For example, Edward and Alphonse could enroll in Ouran High School. Edward's short temper and Alphonse's gentle nature would create interesting interactions with the host club members. They could get involved in the club's activities, like the commoner's day event, and bring their alchemical knowledge into this new world.
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.
For the FMA characters in the OHSHC world, you can show how they adapt to the new social norms. Edward, for instance, might initially be frustrated with the elaborate social games in OHSHC but gradually learn to play along. You can also focus on how their goals change. Maybe they start off trying to find a way back to their own world but then become invested in the problems of the OHSHC characters. For the OHSHC characters in the FMA world, their charm could be used in different ways. Tamaki could try to use his charm to negotiate with the military in FMA.
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.
To write a good fma crossover fanfic, start by choosing the right crossover. It should be something that has potential for interesting interactions. For instance, if it's an FMA and Bleach crossover, think about how alchemy and soul reaper powers could interact. Research both fandoms thoroughly. Look at their cultures, technologies, and power systems. When writing the story, focus on the emotions of the characters. Show how they react to this new and strange crossover world. Use vivid descriptions to bring both universes to life in the fanfic. And don't forget to proofread and get feedback from other fans.
One idea could be that Greed finds himself in an alternate universe where the rules of alchemy are slightly different. He could then try to adapt and use this new knowledge to his advantage, all while facing new and unexpected enemies. Another plot could involve Greed teaming up with characters from another fandom in a quest for a powerful and mysterious object that exists in both worlds.
One plot idea could be that Ichigo somehow gets transported to the FMA world. He might initially be confused by the alchemical rules but then use his Soul Reaper powers in unique ways. For example, his Zanpakuto could be seen as a form of alchemical weapon, and he could fight alongside the Elric brothers against the homunculi. Another idea is that the Philosopher's Stone has some connection to the Hollow within Ichigo, and characters from both universes have to work together to prevent a catastrophic event.
One idea could be that characters from Fma use their alchemical skills in the wizarding world of Harry Potter. For example, Edward could try to transmute a magical object using his knowledge of equivalent exchange. Another plot idea might be that the Philosophers Stone from Harry Potter has some connection to the alchemy in Fma, and characters from both universes have to work together to protect it from dark forces. Also, perhaps a character from Fma gets sorted into one of the Hogwarts houses and has to adapt to the magical school life while still having their own alchemical powers.
Well, a cool plot could be that the Death Eaters are interested in the power of alchemy. They try to capture Edward to force him to use alchemy for their evil purposes. Harry, Ron, and Hermione then rescue him. In return, Edward offers to help them with his alchemical skills in their fight against Voldemort. For example, he could use alchemy to create unique magical items or barriers. Also, Alphonse could befriend some of the Hogwarts ghosts and learn about the secrets of the castle from them, which could be useful in the fight against the dark side.
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 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.