One idea could be that Naruto somehow ends up in the world of Fullmetal Alchemist and his chakra-based attacks are mistaken for some form of alchemical explosions. For example, his Rasengan could be seen as a condensed form of alchemical energy that explodes on impact.
One popular character is Scar. His unique brand of alchemy and his backstory of revenge can create some really intense crossovers. Edward Elric, as mentioned before, is always popular. He can be paired with characters from other series to have some amazing adventures. Hohenheim is also a character that shows up in a lot of crossovers. His long life and his connection to the Homunculus can add a lot of depth to crossover stories. He can interact with other powerful or long - lived characters from different universes in very engaging ways.
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.
A plot could involve the Avengers trying to understand the concept of alchemy. Maybe Tony Stark becomes really interested in it and tries to incorporate it into his tech. Meanwhile, the Fullmetal Alchemist characters are in awe of the high - tech world of the Avengers. They could end up sharing knowledge and skills to take on a powerful foe that neither could defeat alone. For example, a powerful demon from the Fullmetal Alchemist world that has found a way to use advanced weaponry from the Avengers' world.
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 popular theme is the exploration of Envy's complex personality in the crossover. Since Envy is a homunculus with deep - seated issues like envy (obviously), in a crossover, it could be interesting to see how that plays out against new characters or new settings from the other fandom. Another theme could be the struggle for power. Envy often desires power in the Fullmetal Alchemist universe, and in a crossover, this could be magnified or redirected depending on the nature of the crossover. Also, themes of redemption are possible. Despite being a villain, Envy has moments of self - realization, and a crossover could explore if and how Envy could be redeemed in a different fictional context.
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 popular theme is the exploration of Greed's complex character in the crossover setting. Since he has his own unique set of desires and moral ambiguities, it often leads to interesting storylines where he interacts with other characters from different universes or story arcs within the Fullmetal Alchemist world. Another theme could be the power dynamics. How does Greed's power compare and interact with the powers of characters from the crossover? It can create some intense battle or cooperation scenarios. Also, the theme of redemption sometimes shows up. As Greed is a character with a lot of 'greed' (obviously), his journey towards possible redemption or further fall into his vices can be a fascinating plot point in fanfictions.
One possible storyline is a cross - over where the ninjas in Naruto are exposed to the equivalent exchange principle of alchemy. They might start using it in their training, like instead of just relying on chakra, they think about what they are giving up to gain a new skill. For instance, a ninja might give up some of his speed to gain more strength. In the case of the Fullmetal Alchemist characters in the Naruto world, they could be involved in the battles between the villages. Alphonse could use his armor body to protect the villagers from attacks. And Scar from Fullmetal Alchemist could have an interesting interaction with the shinobi, as his unique form of alchemy might be seen as a very different type of jutsu.
Edward Elric from Fullmetal Alchemist would probably be very curious about the unique abilities in RWBY, like semblances. He might try to study them from an alchemical perspective. Ruby, on the other hand, would likely be in awe of Edward's alchemy skills and his mechanical arm and leg. She could see him as a kind of mentor in a way.