East Asia (more specifically, East China), just like the British Isles, look so empty because they have experienced intense human activity and specially industrialization for centuries, there aren't many true natural areas left, and less well preserved complete ecosystems. The deer won't solve that emptiness because is an extinct in the wild species, discovered to science when already on captivity. I think it even shouldn't have a distribution map in Zoopedia, and I don't know how much do we know about its historical range, but I suspect they had been struggling in the wild for long time before its extinction. And then the takin shares habitat with both pandas and some other animals already in PZ, probably it would only increase the intense colour spot in the NE Himalayas and SW China.
(Anyway I would like both species in game

)
Similar things (dense population, severe deforestation) happen with Central Madagascar or Southern Nigeria (e.g.), they are notoriously empty surrounded by dark colour areas... But it would be difficult to find species that solve it.
But then the map clearly shows other areas with remarkable biodiversity but lacking representation in the game or IRL zoos, such as the Caribbean, Java or New Zealand. A wonderful work
@call me Omi !!