Anyways, back on subject, at this point I don't feel like any biome is lacking, now. I feel like we have a good selection.
Continent-wise, there are a few more. North American deserts could use 2 more species (roadrunner and peccary) to compliment the prairie dog, cougar, armadillo, and pronghorn representing the deserts.
South American taiga could use some love, like the Spectacled bear and Patagonian mara. I'd also say the rhea is important for grasslands representation.
Australia is pretty good. For international roster, it's excellent. For a local zoo, needs some more, but I think really it's the Tasmanian devil players mostly want. That's the last big one.
For Oceania as a whole, the kiwi and tuatara are important, with the blue penguin being a bonus.
For tropical biomes worldwide, it's good. Really, the key missing species are red river hogs, primates, birds, and a sloth.
That's about all I can remember at the top of my head. Am I missing anything?
This is of course all just my opinion, but breaking down both continents and regions, all 6 continents with a variety of animal life are fairly well represented as a whole. Not taking into account flying birds:
North America is probably the most well represented overall, something we couldn't have said 15 months ago at all. The Biomes that it still needs some representation for are the Tundra and Desert. I like your suggestions for the desert and then adding in the Muskox, Walrus and Wolverine for Tundra.
Asia is the next well most represented. Again, the Middle East area could use more representation, and so could Northern Asia, but that again brings in both deserts and tundra.
Europe is also very well represented at this point. Again more Tundra animals would be good additions, but overall the biomes are covered. The Wild Boar seems to be the animal being brought up the most now. The European Brown Bear would be a nice modification to the Himalayan Brown Bear in the game now, but I'd rather it just be changed and not have an additional animal included. Also the Wolverine and Muskox would include European Tundra representation, and the Wild Boar and EBB would include both Asia and Europe, so I'm not even sure we're looking at any animals that are representing Europe only.
Oceania is a funny case. Before the Grasslands pack I would have put it at the top of the list for needing representation, and now it's slipped down to third place. Funny how the addition of two animals makes such a huge difference, but they are staple animals. We have good biome representation here. Most of the comments for more animals center around the Tasmanian Devil, Kiwi and Tree Kangaroo, so really, it's a matter of wanting very specific animals, not just more regional representation in general.
Africa is always going to be a conundrum. It's one the most represented continents in the game and in zoos in real life, yet at the same time is usually the continent that is needing more representation. I guess because African animals offer more zoo staples than any other continent, and a few continents combined. Aside from North Africa (again, the desert) I can't say the continent isn't well represented, it's just a matter of putting in even more animals from each boime.
South America is, to me, the continent needing the most representation, for all the biomes, not just tropical. There's just a lot of South America that isn't represented well at all. It seems to be more of a thought that South America should be all about the Amazons, with little thought to non-tropical South America. The Maned Wolf helped in that regard to give Grassland South America some representation, but we need more! Also do need more Amazonian representation, kind of goes against what I just said, but it's also one of the most bio-diverse regions of the planet. The game doesn't really reflect that.
So effectively, biomes needing more in general:
Deserts and Tundras. Of Asia, North America and Europe (for Tundras anyways)
All biomes for South America.