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.
Ichigo and Edward Elric would make an interesting pairing. Ichigo's brash and impulsive nature combined with Edward's hot - headedness yet intelligent approach could lead to some great interactions. They could learn from each other's combat styles and ways of thinking. Also, both are on a journey to protect something - Ichigo his friends and the Soul Society, and Edward his brother and his quest for the Philosopher's Stone.
Often, these fanfictions blend the dark and complex world of Fullmetal Alchemist with the idea of 'lust' which could be in terms of a character's desires or a new, alluring force in the story. They might also explore different character relationships and how 'lust' can either strengthen or disrupt them.
One common theme is identity. Since the homunculi are in a new crossover world, they often question who they are and their place in this new setting. For example, in many crossovers, they might compare themselves to the new characters they meet.
Some fma crossovers involve characters from DC Comics. Picture Batman or Superman in the world of FMA, dealing with the concept of equivalent exchange and the political intrigue in that world. It can lead to some really unique storylines and character interactions.
One interesting character to crossover with Ichigo is Light Yagami from Death Note. The clash between Ichigo, who is a protector, and Light, who is a morally ambiguous character with the power to kill, would be very engaging. Saitama from One - Punch Man could also be a great choice. Saitama's overpowered nature and Ichigo's journey to become stronger would make for an interesting story. And what about Alucard from Hellsing? His vampire powers and Ichigo's powers related to the afterlife would create a really dark and exciting crossover.
Aizen Sousuke would be an interesting crossover character. Since he's so powerful and manipulative in the Bleach universe, seeing him in a different setting with Ichigo could create some really complex and intense storylines. They could be forced to work together against an even greater evil in a crossover universe.
Ichigo could meet Frodo Baggins. Frodo, being from a very different kind of world, would find Ichigo's situation strange. But they could bond over their shared sense of being out of place in some ways. Frodo could teach Ichigo about the importance of perseverance in the face of great odds, and Ichigo could protect Frodo from any potential threats in this crossover world with his swordsmanship.
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.