Well the reason that the Australia hypothesis is so strong is that Frontier seems to be banging off highly requested animals that were purposefully left out of the game (not the most cash money practice but screw it). Polar bears and reindeer in the Arctic Pack and jaguars and giant anteaters in the SA pack. Now, the 2 most heavily requested animals are penguins and kangaroos.
It's not so much that they are 'highly-requested'; Frontier has a roadmap. They already know exactly what DLC they are going to release and when. I think these first three DLC packs though (Arctic, SA, and whatever is next) were probably either already done or close to being done prior to the game even launching (in other words, it's unlikely they started and finished the Arctic Pack between November and December, and then started and finished the SA Pack between December and April).
They've chosen their themes deliberately. Arctic for the winter/Christmas period, which makes sense, South America for the Spring period (rainforests are bastions of natural life, spring is all about nature, rebirth, and so on), and then for the summer, Australia makes the most sense unless they do something completely out of left field (Australia is hot, summery, known for its beaches and deserts and bushfires).