I just saw the post about the Koala birth in Edinburgh. Those who remember him will know how much Frank Wheeler, the Head Keeper of the Clore who passed away in 2006, wanted them back at London. They are exactly the sort of species that the Zoo is lacking.
A sensible suggestion, a rarely seen & relatively high profile species, which doesn't require too much space. London has lost too many of it's crowd pullers and some will never return to the current site e.g elephants, rhinos & sealions whilst others such as bears, more big cats or orangutans don't seem to be on the horizon. Koala might be just right at London