I like this idea and agree it could be used to show just how zoos have changed throughout the decades.
H'mm. London has got to get away from this attitude of being permanently defensive about its history, and imagining that its existing buildings can't be adapted for other animal exhibit purposes.
The Casson Pavilion is the central feature in the Main Garden. To have it closed, except for very good reasons indeed, to the public is (and I apologise for causing offence) frankly ridiculous. As has been pointed out earlier, a den designed to hold an adult elephant (and "Dicksie", the larger African cow in 1965, was a very big animal indeed) is more than capable of being used for one or two tapirs that need a bit of extra privacy.
I miscounted earlier; the two pairs of elephant and rhino dens, the respective hospital dens, the mess room and the elephant bath make up twelve bays, not sixteen. Allowing for any newly restored building having enough keepers to make the mess room a necessity, but also amending the entrance on the Events Lawn side to be able to act as both exit and entrance, so that the entrance lobby on the other side of the building can be utilised, that gives an awful lot of potential animal exhibit space.
The units that were installed for temporary purposes when the Clore and the Bird House were being redeveloped have already shown that they can hold birds and smaller mammals. The Events animals could be in the empty Petcare Centre - that would enable (say) Prevost's Squirrel, Belanger's Treeshrew and a chevrotain species to be held. Might the other group of small animal dens be set on a reverse lighting cycle? That would make it possible to hold one or more species of loris, or cloud rats; maybe even pangolins, one day.
I would love to have Orangs back, and Sun Bears. But I'm not sure how easily the rhino paddocks could be adapted to hold them. Maybe a group of Banteng (if the rhino paddock was used as one exhibit space) or two from Lowland Anoa, Visayan Warty Pig and Prince Alfred's Deer (if used as two) might be easier.
One other thought:
Arctictis binturong was first named by Stamford Raffles, ZSL's founder. The Binturong, so well described by Gerald Durrell as looking like a badly made hearth rug, is one of the most wonderfully outlandish mammals to be imagined - a carnivore that is largely vegetarian, where the female is bigger than the male, and possessing a prehensile tail! Space for a pair of these strange animals to slumber away most of the day, to occasionally wake up and amaze visitors, ought surely to be found. And Raffles' bust, so long sitting in the Society's Offices, would surely be pleased to be brought out to gaze down on the interaction of animals and visitors.