It feels wrong to say I don't want more animals of any kind, but mythical is a big no. Now if they added a new equestrian and there was a cheat to make it LOOK like a unicorn in YOUR zoo, but still act the same, that could be cool, but a pack with fairies, dragons, and phoenixes would not be purchased by me for any reason. This isn't that kind of game. Same for extinct things. Even though in theory we might be able to clone stuff that has died out, it hasn't been done (at least not successfully), and it sort of deserves its own game (which it has, actually).
I think any new modern animal would be good, except for subspecies. I don't think we needed two tigers anyway. "Reskins" like a new rhino, a new moniter, or a new crocodile would still be welcomed, but two nearly identical types of zebra aren't something this game really needs, at least not in my opinion.
Now I know I said that extinct things would be bad, but I wonder if our vets could engineer some of our giant centipedes into arthropleura. Maybe that's what goes on in quarantine that we can't seen...