I'm not overly fond of supernaturals, but I do miss witches. A lot.
If they add mermaids again it would have to be with an incredible new feature to get me on board, like actually being able to live under water (including building under water).
I don't know about Werewolves, they did not really do it for me in TS3. I guess they could be more interesting with the system they used in the Vampires pack with more aspects of progression in the life state.
Superheroes would be difficult to implement, I think. I would expect a large amount of powers to chose from, otherwise they could get boring quickly.