I want them all... I'm so weakI agree that three more hoofstock species wouldn't hurt.
I'd personally pick of those: Aoudad, Chevrotain and Nilgai.
- Alpine region I'd choose either Aoudad (its yet another caprine yes but its a hot weather version which we don't have) or Japanese Serow (its just from my personal wishlist...) Or, maybe a Chinese Goral since it is as unique as a Serow but could be found in subtropical forests towards the extreme South of their range.
- I think Elk are near essential. They're beautiful and NA could use a more typical version of Deer than the giant Moose and the Northern Caribou IMO. That said, I'd love Asian Deer. Pere David's Deer has the best/interesting conservation story and is elegant looking. Reeve's Muntjac would add a smaller ungulate which is pretty necessary! Plus its among the most unique deer ever. Greater Malayan Chevrotain is also unique (not even an actual deer) and is tiny and is from the SE Asia forests.
- Blackbuck I think is almost a given. Its elegant and adds an antelope to warm weather Asia. I think an oddball instead could be a Nilgai with pretty coloration and unique build among the antelopes.
As far as non hoofstock animals filling similar niche I think Rock Hyrax is the best!