With Sydney (Blacktown) Zoo actually being a smaller site than Taronga at only about 8 or 9 hectares (there is a roughly 4,400 sq m space behind the African ungulates habitiats which includes a 1,500m2 dam/reservoir, imagine may have been intended for river hippos', would require a new arm of the raised walkway though for visitors to be able to see that area; built over what is the dividing area between Tino the rhino + the male nyala and the giraffes, zebras and female nyalas, + ostriches).
If was ever put into use, small for hippos, though its about the same scale as the other exotic megafauna mammal's exhibits based on their size, probably could never have more than 3 hippos there, even just 2). So yeah the idea of a actual open range zoo in western Sydney (as we all know, a concept going way back many decades) will most probably never eventuate.
Often wondered if Hunter Valley Zoo/WP could have become an open range zoo that would be accessible for not only greater Newcastle/Hunter region and even Central Coasters, but also possible as daytrips for Sydneysiders (if was possible and not financially impossible to acquire/had acquired neighbouring acreages of land to expand the site of).
Of recent years, us Sydney lot are once again lucky with opportunities to see cheetahs at three different places: Sydney-Blacktown, Symbio, and Wild Cat Conservation Centre if one can book the private visit and has the car fuel (which also means getting to see clouded leopards, caracals and servals also). Except for a brief stay by some young cheetahs from Dubbo in 2001 it's been a long time since had cheetahs living near Sydney until the afformentioned three started keeping them. With river hippos and other large animals, it's tough without anything of an exceptionally large site zoo within an hour of Sydney (though Sydney-Blacktown may still try, will be hard to do and house them satisafactorally though).
Very true about leopards and other arboreal cats
@Younesmark. Can't wait to see Chaturi at Mogo finally (and Phoenix at Zambi, he seems magnificent too). Personally long wanted Taronga to return to holding leopards again (Sri Lankan in this case) or clouded leopards, ever since the snow leopards were sent away. Think you guys are spot on about Taronga and Sri Lankan leopards.
edit: actually 2,000 sq m reservoir dam on the north-western corner of Sydney Zoo (there appears to be a rocky isthmus 'path' blocking the furthest 500m2 of it). Also some of that area has shallow water depth from the dam so its definitely much more a waterbody space than a land surface space looking at again.