Some I'd like to see, even if not common in zoos or in any at all, in the wild at least.
Mountain gorilla
Snow leopard
Asiatic lion
Maned wolf
Indri
Moose
Cape buffalo
Honey badger
Wolverine
Giant forest hog
Baleen whales (especially blue and humpback)
Grizzly bear
Pallas's cat
Bat-eared fox
Andean bear
Sperm whale
Leopard seal
Thorny devil
Musk ox
Narwhal
Asiatic black bear
California condor
Lowland streaked tenrec
Great white shark
African forest elephant
Leatherback sea turtle
Gelada
Mako shark
Black-backed jackal
African fish eagle
Peregrine falcon
Fox squirrel
All billfish
Bull shark
Mud snake
Javan rhinoceros
Sumatran rhinoceros
Dragon snake
Bateleur
Eurasian red squirrel
Pronghorn
Dugong
Emperor penguin
Basking shark
Mountain goat
Ethiopian wolf
Brown hyena
New World tapirs
Ocean sunfish
Tuatara
Blue-ringed octopus
Box jelly
Aardvark
Kakapo
Striped hyena
Aye-aye
Thresher shark
Any loris
Giant salamanders
Tarsier
Aardwolf
Kiwi