Fairies.
Also Mermaids.
So many possibilities.
So much inspiration.
Can't choose both.
Werewolves are nice to have, but could go horribly wrong. TS3's werewolves were the largest disappointment of that pack. Please, at least have them turn into actual wolves... not hairy sims with strong lower jaws.
There's also the problem of divorcing them from vampires or witches.
Robots are cool. We'll probably get them at some point, even if it's part of a larger pack.
Genies are alright. I can see how they could be improved... think of all the ancient mythologies to draw from! Okay, well maybe it's better to stick with Aladdin and I Dream of Genie. Mythical genies are probably the scariest supernaturals in the list.
Imaginary friends could also go badly, and I know many people are strongly against it, particularly if it would be given it's own pack. Still, it's also likely we will see them eventually in some form.
It's unlikely that we'll see revamped ghosts and plantsims, and I'm pretty sure the gurus addressed the issue at some point.
Zombies are very unlikely, and the general belief is that they could never truly be done justice in anything besides a post-apocalyptic spin-off.