You are absolutely correct, ungulate Asian species would be amazing to have, the once you mentioned alongside Chital Deer, Sambar Deer, Barasingha Deer, and of course my beloved Gaur (I need him back) but we still hit the wall of the four pack species every single time, maybe even more so that in Africa with Asia, since you have a very large amount of popular small carnivores, primates and larger cats that will surely take precedent. No matter how you slice it, we are looking at an immense amount of species being left out of the game, with the current model. The saddest part is that the general consensus among a lot of the players here, is that this not only will not change, but that asking for it, is pointless, so with this defeated mentality we have zero chance of ever seeing this magnificent animals.
I'm not sure that's so accurate - looking at Frontier's other games, DLC types seem to vary quite a lot. To me it entirely depends on what "level" of DLC the first three packs were.
Comparing Zoo to Coaster, Coaster's first three big updates were the Winter, Spring, and Summer Updates, which added limited new content. Then came the Spooky Pack, which was much more significant and included new gameplay features. They also included ride-only packs which didn't come with much in the way of scenery and building, but included, well, new rides.
So there is a chance that the Arctic, SA, and Australia Packs were "low level" updates, and that there is more to come. We can never say for sure, however. Frankly there are endless possibilities within even the current DLC framework to get a good chunk of the animals we want, depending on how they theme their DLC's. Basing them on continents might not be the MO forever - looking at Africa especially, we already have a very well fleshed-out African building and scenery theme, so adding another one might be unnecessary. So what do we get instead? An Africa Pack with no building set? Or something else entirely?
I have a feeling that the continent-themed packs were created primarily to cover the 'gaps' in the roster in terms of not only animal diversity, but also in terms of building and scenery items. Going forward, now that those areas are covered, it's possible we'll see something new in play. Maybe something bigger, like PlanCo's Spooky Pack (when compared to the smaller updates), or something smaller, like the ride-only packs or the JWE Dinosaur packs (essentially more "deluxe upgrades", if you will).