If I had 10 years to run the zoo, these would be my 10 priorities, mainly focusing on finding a use for empty areas.
Most of these should be somewhat realistic. In an ideal world, London would have Zürich levels of investments from city-based businessmen and could expand into the nearby park, build a world-class rainforest house and acquire some manatees and elephants, but none of that will ever happen in the short-term. That said I'm not taking this too seriously so I won't be considering the availability of stock for certain species etc.
1) Reopen to the North Bank of the Regent's Canal, though the existing aviaries (if they are even still standing) will need drastic renovation as they were incredibly derelict. Historically, there were three bird exhibits here: a pheasantry, an owlery, and a crane and goose paddock. I would bring back the pheasantry with an emphasis on the titular pheasants as well as passerines, but larger birds such as hornbills and owls will be avoided. I will also create a Prague-esque walkthrough for native species with (much like the Prague aviaries once did) hedgehogs and native reptiles dwelling on the ground portion. Finally, instead of the crane and goose paddock, I would construct a cassowary paddock, which given the flightless nature of the bird would not require clipping or pinioning.
2) Permanently open the Cassons (I have no idea why they keep closing and reopening them). In keeping with the Southeast Asian theme of the area, I would construct various terrariums that could hold reptiles and small mammals from that corner of the globe. A slightly more fantastical addition (which may be more plausible than it seems given recent breeding success) would be pangolins, which for whatever reason I have often dreamt about going on display in the Cassons. Again helping with the S.E.A. theme, move the Babirusa over to the RRH paddock and bring back Malayan Tapir for the current Babirusa paddock. That said, I do love seeing the RRH at London so I'm not sure what I would do with them. I know some American zoos mix them with gorillas so they could go in the expanded 'Gorilla Kingdom,' but I also know many zoos have struggled with gorilla mixes in general over the years.
3) As has recently been suggested and debated in the news thread, bring back the Giant Anteaters and Vicunas, permanently acquire Capybara and potentially even Brazilian Tapir (if the paddock is big enough) to create a South American mixed-species enclosure on the empty lawn opposite 'Tiny Giants.'
4) Renovate 'Meet the Monkeys' so it no longer has to be a dead end. If the problem is the rumoured drainage difficulties causing flooding at the latter end of the enclosure, then perhaps a boardwalk could resolve this. This would greatly reduce the necessity of backtracking to get from this area to Blackburn in spite of them being right next door, and would also mean the aforementioned South American zone could continue with the squirrel monkeys and the coatis on the other side of the walkthrough.
5) Sun Bears in front of the Mappins. The smallest of bears, I think the current wallaby and emu enclosure would be more than big enough, though their aboreal nature would have to be catered for by extensive climbing. A nice tribute to the history of bears in the Mappin grottos, and also a way of creating another mini geo-zone with the tigers, gibbons, babirusas and Komodos all nearby.
If I'm not mistaken, Whipsnade's current geographical theming was a gradual process that came about by slowly replacing animals with ones from the same region as their neighbours. I could definitely see this happening at London; in fact, I feel it has already begun.
6) Pinioned and wing-clipped birds is my least favourite trend in modern zoos, and sadly London still does it with the historic Three Island Pond. I don't know if this would violate the Pond's listed status, but if at all possible, I would net it over and find some more interesting birds to mix with the existing flamingos and pelicans, much like what Exmoor has done. If netting it over is not possible, then I would bring over the Sitatunga from Whipsnade and, taking inspiration from Prague, create a swampland enclosure for this species.
7) This one will certainly never happen as the zoo has already confirmed its future plans for the Reptile House (an interactive, educational thing for children), but as it is not listed, I would do something similar to what they did with the Parrot House and demolish everything except the historic entrance archway, which is a gorgeous facade. Again, like the Parrot House entrance now being opposite the Macaw Aviaries, I would transport this entrance to Barclay's Court and have it opposite SLoRA.
As for what I would do with the site of the old Reptile House: a new Aquarium, completely from scratch. Or perhaps pinnipeds, something that I really feel is missing with neither ZSL collection holding them at the moment.
8) Much like the pangolins in the Cassons, this is one that came to me in a dream: Giant Otters in the former Giant Tortoise enclosure. It would need quite a bit of a transformation, but I think it would really work.
9) Completely net over 'Penguin Beach' and hold a myriad of interesting seabirds. Eiders, puffins and guillemots would be my priorities. None of those very realistically attainable, but the bird department has been doing very well in recent times.
10) Bring back the hummingbirds, but not in their former room in Blackburn (which I quite like the current inhabitants of), but instead in the space opposite the Pavilion in their own devoted walkthrough greenhouse.