From someone who wants to make a dog’s dinner oc: how far can you push the science-fiction? Like how close can it get to straight up fantasy as opposed to something grounded-ish? Do you have a hard limit on stuff thats possible in-universe?
To make a long, grueling explanation short: There are almost no limits, but I have a few hard rules I placed for myself!
While the people of Elysium can be superstitious and believe in the supernatural, or lucky charms, ghosts, and monsters, it could never be proven. Magic, spirits, or ghosts are always implied but always have reasonable explanations as to why certain things occurred.
Animal/Human hybrids can exist, but at a cost. They are always seen as an abomination, and they look very weird every time. They're rare because people who attempt it usually die from infection, sickness, or general rot, but some people have pulled it off, and they're horrifying to look at.
Like the rules of a genie, "I cannot make people fall in love with you," but...unlike a genie, "I can bring them back from the dead." Elysium is a horrible, cruel little island. When characters first arrive in the city, they are not treated kindly, but seen as "free game," and permission to "hunt" every character that sits on that isle has passed the first trial of survival.
YOUR CHARACTER WILL BE HUNTED WHEN THEY FIRST ARRIVE
Very few will like them strictly based on charm. Ask yourself what their best trait, are they witty, dexterous, strong, or very clever? Take that best side of your character and twist it into something that'll be awful later down the line. Elysium measures a character's worth based on how useful they are, not how kind or attractive they are.
Everything else, though? Fair game. There are bolters who fly from hunting planes in the sky, blighters who have fear gas tucked away in their pockets made from strange and bizarre plants nowhere else seen on this globe, people who have turned themselves into leviathans and, like sirens, show you your deepest desires just to eat you alive.
My rule? You want to make something crazy? Your character had to work really hard for it.