I'm not sure specifically if there is a publicly available 'fma riza pregnant fanfic'. You could try searching on some fanfiction websites like Archive of Our Own or FanFiction.net.
An 'fma oc fanfic' is a fan - created story related to 'Fullmetal Alchemist' (FMA) with an original character (OC). It could be about the adventures of this new character in the FMA universe, their interactions with the existing characters like Edward and Alphonse, or how they might change the course of the story's events.
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.
The unique alchemy system in FMA can be incorporated into a family portrait fanfic. For example, the transmutation circles could be related to the family bond, like a special transmutation circle that represents the family. This would be different from other fanfics.
It could be about the Elric brothers and their family. Maybe it shows a different take on their family before the tragedy. For example, it could focus on their daily life, with Hohenheim being a part of it, and how they interacted as a family. It might also include their mother's influence on them.
One possible plot could be that Riza discovers she's pregnant during a time of relative peace in Amestris. She then has to deal with the reactions of her comrades, especially Roy Mustang, who might be both overjoyed and worried about the implications for their future.
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.
Well, one interesting aspect could be the crossover of different universes. The RWBY universe with its unique characters and powers combined with the Fullmetal Alchemist: Briggs setting can lead to really cool and unexpected storylines. For example, seeing RWBY characters interact with the military structure in Briggs.
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.
There are also fanfics that explore alternate universes. In these, the relationship between Ed and Al could be different from the canon. Maybe they are in a world where alchemy works differently, or they have different personalities. This allows for a lot of creativity in exploring their relationship in new settings and scenarios. It can also lead to different interpretations of their bond, whether it becomes more strained or even closer in these alternate worlds.