My usual response to this would be the greater bilby, but given I've talked about them here numerous times before I thought I'd go with another Australian animal that's been rising on my personal wishlist recently - the magpie goose.
We don't have any waterfowl in the game yet, and this is a particularly special species - it's the only living member of the family Anseranatidae, which split off from all other birds around the time the non-avian dinosaurs went extinct and was widespread across the world in the early Cenozoic. Found in northern Australia and southern New Guinea, a region with few species in the game thus far, the magpie goose is downright
essential for representing the tropical monsoonal floodplains that typify this part of the world, being arguably as iconic to the region as the saltwater crocodile. It's also not that uncommon in zoos outside of Australia, with 35 holdings in the EAZA and, from what I can see, a decent presence in American zoos as well.
Unfortunately, sitting at the 329th place on the forum metawishlist, its chances of being added are extraordinarily slim. The pack that would probably have been best suited to contain it, the Wetlands Animal Pack, has also already come to pass, with perhaps its only remaining chance being an oddball inclusion in a dedicated Avian Animal Pack.
I just… I love them lol
