Ok, thought I'd have a go at this one with 2 lists. One hopeful, one total fantasy, made up of species I know we will never get to see.
Hopeful
-Gharial
-Tasmanian Devil
-Hartebeest
-Red-knobbed Hornbill
-Wreathed Hornbill
-Kodiak Bear
-Ragianna Bird of Paradise
-Northern Cassowary
-West Indian Manatee
-American & Orinoco Crocodile
-Baird's Tapir
-Walrus
-Bearded Vulture
-Brown Hyena
-Shoebill
-Golden Snub-nosed Monkey
I had put Tree Kangaroo on this list, but then remembered that Belfast & London keep them. Would be nice to see other collections keep them, though.
I'd also like to see more collections with Asiatic Black & Sloth Bears, continuing the recent return of bear species to UK collections.
Fantasy
-Amazon River Dolphin
-Philippine Eagle
-Sumatran Rhino
-Bontebok
-Amazonian Manatee
-Hoatzin
-Giant Forest Hog
-Giant Armadillo
-Kakapo
-White-rumped Vulture
-Greater Adjutant Stork
-Helmeted Hornbill
-Pangolin
-Blue Bird of Paradise
-Jentink's & Yellow-backed Duikers
-Indri
-Proboscis Monkey
-Douc Langur
-Yellow-eyed Penguin
I'd better stop there.