Even that could be done by applying moderate changes to the sets we already have to make them look rustier and "Outbackish." It ain't like with the Arctic DLC where they made what's basically a Skyrim building set.
At their core all the building pieces are basically identical, only with new skins and textures applied. There's no real difference between the Arctic 4m Wall, Mud 4m Wall, Brick 4m Wall, or whatever else (glass seems to work differently since guests can see through it). The additional decorative pieces (such as the half-walls, trims, decorations, beams, and so on) are more uniquely made. It would be the same with an Outback theme. The walls and roofs would just be reskinned from the existing 'template' I'm sure they use for all building pieces, but they'd design some new scenery items, structural props, and so on to go along with it.
So there'd probably be some kind of new corrugated metal roof and wall set, some kind of new type of wood, maybe even something similar to the bamboo except with more basic sticks like a bivouac lean-to or something. Plus there'd need to be the aforementioned Aboriginal art pieces, possibly a new rock set (red desert rocks are pretty Australian), maybe even some more 'basic' scenery pieces since everyone's been requesting those.
I'd like more animals as much as the next person, but I'm examining the fact that they definitely could add a whole new building set if they wanted to, specifically to manage my own expectations. If the first three
Planet Zoo DLC's line up with the first three
Planet Coaster updates (December, April, June), then it's likely that whatever comes next will be the same as the first two in terms of content with maybe a little twist (
South America Pack gave us a new exhibit animal which we weren't expecting, but we knew there would likely be only four habitat animals and a big themed set).
Of course, we can't absolutely guarantee that Australia is coming next, but it does seem likely (Australia is a summer destination, June is the Northern Hemisphere summer where Frontier is based, Arctic was in winter, South America/rainforest was in spring, it all makes sense). I just don't want people to get their hopes up for more and then complain when their expectations aren't met like they did with the SA Pack.