Witches, but actually I want a variety of spellcasters and different types of magic. I have a thread going over in the ideas section with bunch of suggestions in it.
Normally I don't care much for mermaids but if they gave them a little more grit and detail, I could like them. I'd like to see them have a Siren song and the ability to daze Sims and cause them to drown or enslave them or sink their boats. It would be cool if they could have sharp teeth and some creepy options and varieties in CAS like tentacles and jagged fins, as well as beautiful and colorful selections. They should be able to eat raw fish and have ocean pets that they command and be able to gather types of shells and seaweed and rocks to decorate their underwater homes with and create new objects. They should also be able to enchant themselves to walk on land for a limited time and imbue Sims to temporarily breathe underwater and teach them to improve their swimming. They should have giant oyster beds and kelp curtains and coral mazes and underwater fissures that they could utilize. I'd like to see them make use of fish netting and sea glass and sunken ships. Maybe they could even make a Sim rich by leading them to a sunken lost treasure for a favor.
There are lots of possibilities with any mythical beings. I want them all, lol.