Very soon I intend to have my sim (a witch who has mastered the love charm) start a collection of supernatural babies by trying for a baby with various supernatural sims. Vampires, werewolves, fairies, ghosts, the reaper, witch, alien... can she have a mummy baby? And growing a plant baby. And can she get a mermaid baby if she doesn't live in Isla paradiso? She'll probably not have to try for a witch, she'll most likely end up with several unfortunately, as she tries for the others. Is there any way to influence the state of the baby? Any hints or suggestions?

Nraas' Master Contoller will let you add and remove supernatural states. It will also allow you to take a pre-existing Sim and put them into Create A Sim mode so that you can edit them ( I've saved a 'generic' imaginary friend turned human and a 'generic' plant sim to the bin so that I can create them in new families but just swapping their supernatural type with Master Controller will be much easier). It will also allow you to pick how a ghost died (which will change their color) in case you want to change that on a ghost baby.

Your witch cannot have a baby with a mummy the normal way but it is possible with Master Controller. My Sim had a baby with mummy from the Great Pyramid.

Mermaids can be conceive in and outside Isla Paradiso.