If we are limited to 4 species I’d say:
Red kangaroo - I find it more striking than the grey kangaroo.
Emu - so common in zoos around the world and easy to animate.
Koala - an iconic Australian species.
Tasmanian devil - an important conservation message.
Others I’d love to see...
Southern cassowary - easy to animate, a stunning and unusual bird but still relatively common in zoos internationally.
Common wombat - I think this would be very cute in game.
Red-necked wallaby - very common in zoos so a useful addition to the game.
Brolga, Australian pelican, cape barren goose, black swan and other Australian wildfowl would also be amazing but as we haven’t had any ducks, swans or geese yet I won’t make a detailed list. I’ve also left out any birds that would need to be housed in aviaries for obvious reasons.
I love platypus but I tend to agree with those who have pointed out that it wouldn’t really work with the limitations of the game as it stands now and I’d rather see other animals which could be more impactful in the game as we know it.