I think full development of london zoo's existing site and whipsnade should be more of a priority than gaining extra land in regents park. the north bank needs proper attention for a start, and im not sure the 2 lawns in the centre of the zoo add much when you're already part of an enormous green park. Ironically they'd probably have made a better site for the pygmy hippos and be next to the gorillas/other african forest species.
Netting over the 3 island pond, as was propsoed in the 2007 masterplan would mean more unpinnioned birds could be kept there, and moving the horticulture section of the bank frees up more space for other aviaries/houses etc. The huge forecourt outside the shop is a bit of a waste of space imho, and could be redveloped for animal exhibits.
I dont think 36, or even 46 acres, would ever be enough to exhbit much in the way of mega fauna and you can make a very exciting zoo for enthusiats, tourists and children by holding a lot more in the way of small primates, lemurs, tropical houses, free flight bird avairies, aquariums and interactive/play areas and a childrens farm where they can touch real animals. Lions, tigers, giraffes, zebras and camels would probably be enough in the way of megafauna - I dont think adding in 2 solitary black rhino would draw any more people in.
In busy london, a zoo with great variety, lots of different animals, smaller more active animals or unusal animals would be as much of a draw as fewer species of hoofstock. Remember meerkats and penguins are currently among the most popular zoo animals. ZSL has whipsnade for keeping grazing hoofstock and the collection there has suffered lacklustre attention in recent years with dwindling numbers of species.
With exception of lions, tigers and giraffes, i dont think there should be any duplicartion of species over the sites - keep zsl for smaller stock but bigger numbers of animals/species, and larger hoofstock/megafauna for whipsnade where they can have space. Both would be able to draw punters on there own merits.