I agree, we should have a mix between the usual/expected species (lion, zebra, boa constrictor) as well as a few rarer ones. But not TOO rare.
Zoo Tycoon 2 added animals like Javan rhino, mountain gorilla, narwhal and goblin shark, which I never used because of how unrealistic those are to have in your zoo.
The aardvark is somewhere between common and rare in captivity, so it's a great inclusion. Also because of it's uniqueness.
Pronghorns are not kept in European zoos, and I believe quite rare in the USA as well, but they are kept in a few collections so it's realistic enough to be added.