So many people have done this rimuru x mha but i haven't seen this Au so would like to say this au is original? So the monsters in the time i was reincarnated as a slime are actually people with mutant quirk(Mutant Quirks modify the body and change the appearance, making their users targets of discrimination. so prety much they can look like mina or koda ). Tempest is a country that only allows people with mutant quirks this ties into when the Kingdom of Farmenas attacks tempest because it was an act against god,Rimuru deemed being open unsafe so he has multiple people guard the entrances . to go along with mha timeline and quirks so its been like 50 years since the first quirk so what if quirkless people were scared that the people with monsterous quirks were also an act against god and decided they were unworthy of living