In my view, London Zoo is seriously undershooting its potential. I don't believe people know how much potential space is being wasted:
1) The two lawns behind the Giant Turtles and the new Reptile/Amphibian House
2) The Casson Pavilion sitting empty
3) The Mappin Terraces, Aquarium unused
4) The Institute of Virology behind the Mappin Terraces, could this be moved elsewhere?
5) The Old Reptile House soon to be empty (I highly doubt ZSL will house anything here as it's not suitable, it will become a museum/education centre)
6) The lawns next to Butterfly Paradise and War Memorial
7) Land of Lions ethnographic areas + hotel
8) "Activity Den" on the other side of Tiny Giants
9) Penguin Pool of course (maybe move to Tate?)
10) The ZSL main building, Library/Archives, and the Lecture Theatre (maybe plan to move some of these next to the car park?)
11) Woodland Walk next to the canal
12) Across the canal, east of Monkey Valley completely unused. Also there's more space west of Monkey Valley earmarked for education.
So I would really appreciate it if they tried to squeeze out more out of the very limited land available because expansion is 100% impossible. The only viable expansion space is the southeast corner near the Readymoney Drinking Fountain. Maybe you could squeeze out another 2000+ square meters there, enough for one animal exhibit.
I feel ZSL isn't even trying anymore. They take London for granted due to the favourable location, massive and relatively wealthy population and they have no incentive to improve their product. They want to find alternate revenue streams that don't involve more animals. They're also worried that making London Zoo too excellent would reduce visitors to Whipsnade.
As for the zoo-goers with happy memories who don't want Mappin or Casson demolished, the next generation don't have any memories associated with those structures. So, in my view they will have to go eventually.
Of course, finances are stretched but creating an ambitious masterplan MIGHT attract some potential donors. But they haven't bothered.
A fun thing to speculate about would be how one could use some of the wasted spaces I mentioned above to house animals and how ZSL could transform them into exhibit areas. Assuming funds were available, what animals could go there?