Banshee are fairies? I thought they were undead horrors in spirit form :s And here I was picturing little Tinkerbell...
Roughly speaking, there already are fairies in the game - they are the little glowing orbs flying around in Sylvan Glade. Walk up to them and you can make out their little form and hear them laugh playfully.
On a similar note, werewolves are present in statue form and can be heard howling in Forgotten Hollow at night. (Unless you have decided those howls belong to regular wolves)
Anyway, I wouldn't mind a lot more magic in The Sims 4 at all. Its already cartoonish feel is so attuned for it.