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.
Edward Elric and Tony Stark would definitely have some interesting interactions. Edward, with his knowledge of alchemy, would be both intrigued and a bit put off by Stark's high - tech gadgets. Stark, on the other hand, would probably see Edward as a brilliant but somewhat naive young scientist. They could argue about the best way to solve problems, with Edward pushing for more 'natural' alchemical solutions and Stark relying on his tech.
One interesting plot twist could be the discovery that the Philosopher's Stone in the Fullmetal Alchemist world has a connection to the chakra system in Naruto. Maybe it could be used to enhance or disrupt chakra flow in a way that no one expected. Another could be a character from Naruto, like Naruto himself, accidentally using alchemy due to some strange cross - world magic. This would completely change the dynamic as he tries to figure out this new power while still having his ninja abilities.
One interesting plot twist could be the alchemy of the Fullmetal Alchemist world interacting with the Dust in RWBY. For example, maybe an alchemist discovers a way to transmute Dust into something completely new and unexpected, which throws the balance of power in Remnant (RWBY's world) into chaos. Another could be a character from RWBY somehow getting involved in the Philosopher's Stone plot of Fullmetal Alchemist, leading to a race between different factions in both worlds to either obtain or destroy it.
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.
A crossover could involve the Homunculi from Fullmetal Alchemist being a threat that the Avengers have to face. The Avengers would be challenged by their unique abilities and the complex nature of the Homunculi. The alchemists might then team up with the Avengers, sharing their knowledge of the Philosopher's Stone and its power, which could be crucial in fighting the Homunculi.
Writers should first understand the core characteristics of Envy. Know that Envy is a very vengeful and envious character. So, when creating a crossover plot, make sure Envy's actions are driven by these traits. For example, if the crossover is with a fantasy world where there are powerful magical items, Envy might be envious of those who possess them and try to take them. Also, keep the rules of both universes in mind. If in the other universe, magic has certain limitations, don't break those rules when Envy interacts with it. This will make the plot more believable.
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.
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.