Werewolves: Yes, they are mentioned in the story, but we never actually met any children of the moon. The wolves of La Push are shape shifters and that gets confused all too often.

Witches/Wizards: Because there are vampires that have special powers once they are changed, it only makes sense that there would be actual witches. Take the twins, Alec and Jane for example. Both of their powers were very prominent before they were changed. I would like to think that their are humans in the twilight world that are capable of performing spells and just haven't caught the attention of the Volturi.

Nymphs: We don't really know the origin of the vampires of this series. Because of the vampires alluring nature, it is plausible that they descended from nymphs or were created by the hand of other creatures.

Pixies or Fairies: Although Alice is commonly thought to embody the ideal look of a pixie, actual pixies would have done the story good. Had they been in the story, there would have been an actual magical feeling in the twilight world. Even thought the story is supernatural, having these would have made the story more than a two sided view.

Ghosts: This would have added a deeper element to the supernatural aspect of the story. Imagine the real reason Edward was so adamant about keeping Bella alive was because he was haunted by the ghosts of innocent girls he accidentally killed. Now, that would have been an excellent twist. And imagine if only shapeshifters, vampires, and werewolves could see them.

Other Shapeshifters: Imagine us getting to see other tribes that were capable of changing into woodland creatures. Bears, Birds, even snakes. Then imagine if their phasing were to be triggered by other supernatural creatures. Pixies vs. Birds. Werewolves vs. Bears. Fairies vs. Snakes.