You could create an OC who is a journalist in the Fullmetal Alchemist world. Their goal is to expose the truth about the military's use of alchemy and the dark secrets behind it. They start investigating the Elric brothers' journey because they suspect there is more to it than meets the eye. This OC could add a different perspective to the story as they are not directly involved in alchemy but are affected by it. Also, an OC that has the ability to reverse alchemy, which is unheard of in the Fullmetal Alchemist universe, could be really interesting and cause a stir among the existing characters.
One idea could be an OC who is a foreign alchemist with a unique form of alchemy based on their homeland's culture. For example, an alchemist from a desert region who uses sand-based alchemy. Another could be an OC that has a special connection to the Homunculi, not as an enemy but as someone trying to understand and redeem them. Maybe an OC that was experimented on by the same people who created the Homunculi and has some shared traits.
An interesting OC for a Fullmetal Alchemist fanfiction could be a young alchemist prodigy. This OC could have mastered alchemy at a very young age due to a traumatic event that forced them to quickly learn self - defense through alchemy. They could enter the story by challenging Edward or Alphonse in an alchemic duel, believing they are the best. Or perhaps an OC who is a descendant of one of the great alchemists from the past, and they are on a quest to uncover secrets their ancestor left behind, which gets them involved with the main characters.
A practical answer is Chapter 194, because it turns the idea into a concrete scene. The chapter opens with Director Sun, this isn't another one of your production team's wicked..., adds pressure through What caught everyone by surprise, however, was the arrival of this person—, and ends on Wen.
If you want the clearest chapter reference, go to Chapter 148. Early on, Exiting the cottage, Director Sun picked up the phone.; later, the chapter leans on Before he could even greet the caller, Su Yinwan's anxious voice rushed....
If you want the clearest chapter reference, go to Chapter 198. Early on, He wrapped his arms around himself and stepped back half a step, eyeing...; later, the chapter leans on Is that not okay?" Jiang Yan: "....." "Just be normal, please..
If you want the clearest chapter reference, go to Chapter 151. Early on, Now it was Zhang Cheng's turn to be confused.; later, the chapter leans on The problem lies with He Yun?. By the close, I will have my assistant pay the bill., and the chapter closes with Was he...
The best chapter pointer here is Chapter 166, "Daniu Gets Jealous!". It opens with Only after everyone in the small courtyard had left, did Zhao Lang stride... and closes on However, after interviewing the villagers, no one has gone missing, so we....
Look to Chapter 172 for the clearest answer. It begins with Not only Shu Yan, but the entire internet was shocked when they saw Su..., keeps building through Damn, what brand of make-up is Su Yinwan using?, and lands on Where's the chaotic scene I was anticipating?.
Look to Chapter 199 for the clearest answer. It begins with Damn, the production team is actually being so generous this time, giving..., keeps building through He only got one point after catching more than a dozen fish for granddad."..., and lands on You knew all along?" Zhao Lang adopted a.
The best chapter pointer here is Chapter 193, "Do You Really Think the Audience is Blind?". It opens with Jiang Shixu's eyes lit up when he heard there was a gift. and closes on Jiang Cheng was a prime example..
A practical answer is Chapter 196, because it turns the idea into a concrete scene. The chapter opens with Wen Jiaojiao was on the verge of a breakdown., adds pressure through If you all go off to learn fortune-telling, who's going to act for me to..., and ends on For example, Shu Yan drew the.