The first one that comes to mind for me is Central/North Asia. I know there's not a whole lot of biodiversity going on up there but it's worth a few good habitat animals. The next one would probably be the Middle East/Sahara region. I know we already got a pack technically, but only 2 (1 habitat, 1 exhibit) were from there. Other than that, I guess just Australia, South South America, Eastern North America, and maybe the Arctic can use a few more animals.
Middle East and the large portions of northern Asia (such as Mongolia/Gobi Desert).
I think the inclusion of some frequently requested animals, such as Arabian oryx and the Przewalski's horse would help there. Also, if we want a leopard subspecies, there is the endangered
Anatolian leopard (though I know some folks have also suggested the critically endangered Amur leopard). We are also light on exclusively desert species from north America (though cougars and pronghorn are found in deserts, and prairie dogs extend down into New Mexico). I still want an armadillo and a javelina species, but I think I'm unusual there
New Zealand is also completely absent, though the fact that (aside from marine mammals) their native fauna are nearly all birds (with a just a few bats and small reptiles) probably explains this. If we ever do get aviary birds, I hope we get at least a couple from New Zealand. The tuatara is probably the most distinctive reptile in the world (and one of the rarest, certainly), but they aren't something one sees in many zoos outside NZ itself. They do have
some at San Diego, I think, but they are a breeding colony and are not on display. Of course this game also has pandas widely available for our zoos, and they aren't in very many outside of China either. So maybe tuataras should be an exhibit animal at some point?