The trio I would pick would be:
- Eastern Grey Kangaroo - A grey kangaroo would be the simplest macropod to model from the Red Kangaroo (aside from the Antilopine Kangaroo/Wallaroo, but they're very rare in zoos). Mostly requires some adjustments to the head to make it less mule-like, a few small body fixes, a retexture and a thicker coat of fur. Arctic Wolf level stuff basically. Gives us macropod representation for wetter and more temperate environments.
- Sumatran Tiger - Really simple, just scale down the Bengal Tiger and make a few minor changes. Would be greatly appreciated by me at least due to their greater prominence in Australasian zoos, plus the fact they're the most distinct of living tigers.
- American Black Bear - Although I would prefer a Spectacled Bear, I would prefer more work be done on such a unique animal (similar to the Sun Bear). The American Black Bear would, like the tiger, be a really simple job yet also one that would make a lot of people happy.