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.
First, do your research on both RWBY and FMA Briggs. Know the characters, their abilities, the settings, and the rules of each universe. This will help you create a more believable crossover. For example, understanding how alchemy works in FMA and how semblances function in RWBY.
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.
Julia Briggs could be the main character in the story. Maybe she is the one who has an encounter with the ghost or is somehow connected to the events related to the ghost.
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.
One common theme is friendship. The oc and Callie Briggs often start as strangers or acquaintances and develop a deep friendship as the story progresses. Another theme could be adventure. They might embark on exciting quests or missions together.
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.