To wit, here's the start I got on the first one: a skull:
(Note that this is just geometry - I haven't started texturing the skull yet.)
A skull is by far the most-requested new head, so I figured I'd start with it. The nice thing about skulls is that animating them is very easy, since there are no moving parts except the jaw, which is rigid. Even if I decide to make the bone material animateable for expressing emotion, it will still be pretty simple animation. And I can dress up the skull easily with accessories like burning eye sockets, vampire teeth, a gothic crown, demon horns, etc. without changing the base model.