chosen from for example orangutans, bears, large spotted cats, large crocodiles, common hippos and rhinos.
Casson Pavillion has space for perhaps two of these, plus several small animals to complete whatever theme is set there.
Orangutans, sun bears, macaque/langur troop and 3-4 small mammals and reptiles would nicely match the South Asian theme of nearby Sumatran tigers.
Its worth noting London have exhibited all of these species you mention in the past, some quite recently, others, like Common Hippos, not for many years now.
Rhinos-The decision was taken in the early 1990's to remove all the Rhinos and Elephants to Whipsnade, as the building afforded insufficient space for them. I'm sure they will never return to Regents Park.
Bears- returned to Regents Park after a period of absence when the Sloth Bear exhibit was opened on the Mappin Terraces. The bears were not a satisfactory exhibit and later moved to Whipsnade, I doubt they will try larger Bears again after that, though sun Bears might be an acceptable compromise.
Orangutans- they left the Sobell Pavilions about the time the Zoo was in financial crisis(early 90's) as they were thought 'labour intensive' compared to the other Apes and the least satifactory species as an exhibit. I can imagine with its height and vaulted ceilings the Casson House could be adapted for them however and would make a better area than the Sobell Pavilons which were constructed/designed for Primates(comparisons here with Bristol Zoo where the Gorillas live in the converted Elephant quarters and it makes a more satisfactory exhibit than the previous purpose built Ape House was.) If Orangutans were to return to London I hope they would be Sumatran too- numerically at a low ebb both in the Wild and in UK zoos, far more active and so a much more interesting exhiit than Borneans make.
Large Crocodiles- I noticed their absence on a recent visit- something I certainly think the Zoo should address.
Spotted Cats- (Asian) Leopards could quite logically be accomodated in the Casson Pavilion and would complement the nearby tigers.
I agree the building needs a theme and with the planned proximity of the Tigers, South East Asia seems to be the obvious one for them to adopt. There are still the four large main 'Bays' for housing in the Interior and I would suggest each of these four was devoted to one main species(or mix of species) from the following;
Sumatran Orangutans.
Sun Bears.
Malayan Tapirs.
One group of either Langurs/Macaques/gibbons.(they have existing examples of all three in the Zoo already)
Hornbills/ Asian birds.
Leopards.
Outside enclosures would be designed according to species kept, but several seperate exhibits could be made on the available landspace.
...and get rid of the Bearded Pigs/Anoas etc which are non-exhibits for the public.