I'm sort of playing devil's advocate here, because I'm sure your suggestion could be implemented in an excellent way, but I think part of Whipsnade's charm is its park-land feel. The woods and spaces between enclosures are for me a defining feature of the place. If these spaces were replaced with enclosures then it might well be an excellent/better zoo, but I can't help feeling it would lose something else in the process.
That being said, my opinion is based on one visit this year and perhaps you and others remember a larger collection in years gone by. You also make some brilliant points about Black rhino. Whipsnade should absolutely add several 'zoo-flagship' species where captive breeding really could be the difference between extinction and not. Amur leopard etc.
The other thing I'd like to see, and to be honest I think is a necessity, is new housing for the Sea lions and Common hippos.
Sorry (and nothing personal) but this line of argument is having it both ways. If people say that London can't keep as many species as it used to do because the animals all need more space (an argument that some feel has thrown out the metaphorical baby with the metaphorical bath water), it is rather odd to then keep large areas of Whipsnade undeveloped because you like to see big empty spaces.
There is also the fact that as Pertinax points out large chunks of the park have been abandoned in the last thirty years. The cat enclosures near the Brown Bears spring to mind, and the old Lion and Tiger enclosures on the Downs really only needed modern fencing and housing; the actual animal space was excellent. The Lions had far more cover in the shape of bushes in their old chalk pit enclosure than they do now, and personally I think they made a far better exhibit.
Spectacled, Kodiak and Polar Bears all inhabited enclosures that were unquestionably substandard. They might have served other uses - both Chester and Edinburgh have created excellent aviaries for large raptors (Andean Condor and Steller's Sea Eagle respectively) simply by roofing over the pits. Dudley has used one of its old bear pits to house Sulawesi Crested Macaques, a trick that I think has also been performed at Newquay. I know hibernacula for bats have been installed in these spaces: it is hard to imagine these couldn't have been created elsewhere.
I would argue that the more species kept - and of course kept well - the more people will want to return to both sites. Panthera's comments make it clear that a great many Whipsnade visitors were not happy to see so few animals in such a big space. Whipsnade entry prices are not cheap, and children in particular quickly get bored of going to a zoo where they perceive nothing ever changes.
Finally, ZSL has a responsibility to the wider zoo community. Within Europe, taxa such as Andean Bear, Hartmann's Mountain Zebra and Addax - all animals of conservation importance - need further collections to hold stock. It really makes little sense to have a zoo where there is 150 odd acres unused.
One further point: in complete agreement with you about Common Hippo. Whipsnade have made woefully little of this iconic species over the years. Go to West Berlin to see what can be done with it!