I think what's important to consider for 5-animal DLCs is the presence of at least one high-demand, charismatic, either large and somehow unique animal for the cover. Without one of these, I don't expect high sales for a DLC with only 5 animals (even though these packs may be loved by the active community including myself). Of the remaining animals, the capybara, leopard, gibbon, baboon, Przewalski's horse, sloth, walrus, manatee and pelican could play such a role.
In addition, such a DLC will need 150+ scenery pieces and foliage. While I can imagine this for some regions (Middle East, Southeast Asia) and perhaps for mechanics/behaviours (aviaries, aquariums, nocturnal animals), I think it's less likely to see biome packs with a lot of scenery. Off course, they could decide to include tonnes of foliage or rocks instead, but this also seems unlikely. Since I also don't expect 4 animal DLCs in one year, I expect that at least two of the next DLCs will be region-based.
For the animal DLCs, a lot of options are open. They can be related to mechanics (arboreal, aquarium, aviaries, etc.) if the free update include necessary changes to incorporate these animals. Here, it is also possible to have DLCs focussing on biomes or regions and the criteria for a charismatic cover animal may be less high since the pack will contain many animals anyway. Even though we had DLCs for South America, Australia and Africa, all of these regions have plenty of animals that would fit an Animal DLC. For the realism aspect of zoos, this is actually required if we want to have a bit of a meaningful South America or Australia section in the zoo. Off course, these animals can also come in biome packs, which have the added benefit that they can include animals from regions that would otherwise be excluded. I personally don't see potential in a Madagascar DLC (no large charismatic animal and a much smaller region than others), but I can see a fossa or a sifaka appear in a rainforest or islands DLC.