I'd like to see it be Sydney, just because I live there and it would be useful for me.
However, I think it's more likely to be Melbourne first. Sydney is quite a capacity constrained airport (because of both the curfew and the movements cap) so getting decent slots there is quite hard compared to Melbourne.
I'm also curious about which city in Asia they'll use for the stop, contenders seem to be KUL, SIN and CGK, but I believe that DPS has also been mentioned as a possibility in the past.