I’m working on something similar that isn’t based on any fandom but does have my fandom OCs in it, which is why I end up thinking about yours when working on mine.

I remember Cecilia couldn’t be a merfolk because any fish that suited her wouldn’t suit her family, this might be more up for change now honestly with how much I’ve managed to wrangle her from poison related fisalia’s and more into another family for her in my project. Basically I have more experience adapting her family situation without stealing the fisalia’s.

Aprellos one was pretty simple, in which it is not very easy to access trains as a merfolk or build them. And that’s kinda his whole thing, so he’d have to be human to work on trains like he usually does. Though, he could be a merfolk thinking on it now. Like maybe he finds trains underwater, old ones from the disaster that made the world way more mostly ocean, and it inspires baby aprello into a more obsession with learning more. The Ariel of the sea but with trains instead of people, it just looks like a land folk obsession from the outside because trains are only usually up there and so he’s always trying to get peeks and learn how it works. Though, another facet of aprello is that he’s technical, unlike roya who learns from instinct and physical fucking around, aprello learns it very by the book. So I’m not sure how to translate that part hypothetically into merfolk yet.

Roya has to be human because prometheus has to be human, he would absolutely be a scientist in this world working on the new opportunities the mass water thing caused, maybe he used to be affiliated with that group of scientists I remember working on something related to the ocean thing? But I can’t remember what exactly. Prometheus is no help to the peace between species, utterly fascinated by merfolk and the new changes to the world he’s taking any chance to test his experiments. Its not breaking the treaty if he can get merfolk to consent right? (Aka, he manipulates them into taking what he offers Through either naivety or desperation). Unfortunately this does create an issue as usually roya is born with something that relates to his research and “completes” it, but as hybrids are a no go in this world she can’t exactly be born with much related to merfolk, so his research has to involve merfolk but not revolve around it, and instead whatever it revolves around roya ends up being born with a big piece of it. Like he experiments on himself to get close, makes clone nasir and then when he has roya the part inside nasir/prometheus is passed down to her and evolves enough to be important research that would help prometheus with his work. At the time I couldn’t find any way for any of my current OCs to be merfolk or interact with merfolk characters due to the iffy nature between both species, I suppose aprello might be a contender if I can figure out how he’d be book smart when most books can’t survive underwater. And Cecilia if I research fish for her and see if they fit her usual family dynamic I’ve made as a base, and then see if that dynamic even fits into the ocean world without messing too many things up