Personally I don't think it's gonna happen. If planet zoo's lifespan is as most people expect, which is about the same as planet coaster, there's 2-3 packs left. We know Africa's one, North America basically has to be one. The last one could be an aviary pack, but with all the new mechanics that would lead into and a lot of potential difficulties, I think it's more likely they round things out with a European pack (light on the animals but lots of potential architecture styles), a new aquatic pack since despite it's addition, marine animals are pretty underrepresented and those mechanics already exist, or potentially even a fan request pack for the animals people want that were excluded from the packs they should've been in, like capybaras and gibbons.
I could very well be wrong but I think it's more likely they go with one of these options, unless they decide to continue supporting the game further into the future than people expect