Where ever Disney chooses, if they choose to open another park, it is unlikely to be Australia as the figures just don't add up. The problem with Australia is that it would have to be placed somewhere close to a major air hub as the domestic market is not large enough or economically strong enough to sustain a park on its own.
This presents issues issues with land availability etc.
One of the main problems with Disneyland Paris, excluding the fact that its not actually very close to Paris, is the fact that it doesn't attract a financially significantly amount of foreign visitors, the thought was that there would be a huge number of European visitors other than the French, but it hasn't happened that way as it wasn't central enough. The park survives with mainly french & british tourists & very few other European visitors, however the domestic market has proven to be stubbornly low, far lower than was expected, hence annual passes being priced so reduculously low.
All the parks have to be financially viable & profitable 1st & foremost from only the domestic market, any foreign guests are the icing on the cake. Australia definately doesn't fit that category & I'm not convinced Brazil does either, Australia is just too sparsely populated & Brazil whilst populated has a very low average disposable income, not withstanding the tour groups seen at WDW.
The reason parks like Tokyo work is that they have access to a market with an unsatiable desire for their product & a very significant disposable income. China & Hong Kong tap into those same markets.
Looking around the world if I had to make a shrewd guess at where the next park might be built I would pick India or Qatar. They are both largely politically stable & have access to large populations of people, the vast majority of whom have the desposible income required.