I actually prefer it be similar to how they did BTAS origins for clayface where he kept using a specific chemical that allowed him to change his appearance only to get worse and then ultimately destroys his body making him the clay face we know. I think you can get away with doing fantastical characters in a real world setting after all it's all based on comics and we have already got a grounded Batman in Nolan's trilogy so here being grounded to a degree but keeping the spirit of the comics would be nice. Plus we never see these type of villains in film before, a proper clayface, Mr. Freeze, Poison Ivy, Man-Bat, Killer Croc, and Bane would work will if reeves knows how to bring them to life.