I think most of the tropical rainforest animals have been covered with the south american and aquatic dlc's, with the exception of a few like the capybara, coati and sloth. On the other hand, maned wolf, spectacled bear, bush dog, patagonian mara, magellanic penguin, greater rhea, pudú, guanaco, vicuña and any armadillo of choice are all associated with other regions like steppe, alpine or desert. Non-tropical iconic zoo species from south america are the ones that were left out the most in my opinion.Nobody is claiming that? Judging from most wishlists etc., most people want the tropical species from that continent. (and like NZfanatic commented: iconic species).
You mentioned there are many non-tropical species in SA. You are right but this doesn't reflect in most wishlists. And you would pick only non-tropical species from that continent, there isn't that much appeal to it.
If another SA pack should happen, tropical species will be the majority within that pack. And at this point, i'd prefer other biomes.
I wouldn't mind a tropical pack with a mix from several continents somewhere down the line.