Many Chester regulars know each other well, through occasions like the 'Walk and Talks' and other zoo events. We tend to have discussions like this if we bump into each other at zoo, but ZooChat is a very nice alternative and it lets other people join in too
I rather like SMR's suggestions for the round exhibit, although I suppose they could not come about until that area of the zoo is redeveloped.
I wasn't really suggesting a 'museum of Chester Zoo'; I was thinking about things like the family parties near the giant otter pool where you often overhear someone asking "weren't the sea lions kept here?"
Perhaps next year the zoo could put up small 'blue plaques' in the locations of former exhibits, for example on the toilet block between the Roman garden and June's Pavilion the wording could be 'The Site of the Indian Elephant House 1949 to 1962' (or whatever the dates were). They wouldn't need to be large and there wouldn't need to be a large number of them. They wouldn't need to be blue of course, but they ought to be distinctive, they could even have a nice serif font

.
These could be complemented by a dozen or so larger posters in various locations around the zoo - they would need to be positioned carefully to avoid disrupting the theming of the exhibits. For example on the stretch of the wall of Europe on the Edge which used to be part of the polar bear dens, next the plaque about the polar bears, there could be a poster with the heading 'Tigers at Chester Zoo'. It could say that the tigers are now in the new Islands section, but they used to live opposite and give the dates and photos of the enclosure in 1960 and 2010 (for example). It could also mention that Siberian tigers were kept on the site of the Bat House and possibly add something extra, such as the numbers of cubs raised. I would think that there could be about a dozen of these posters dealing with the larger animals, for example one about bears on the fake rock wall at the bongo end of spectacled bear enclosure, one about rhinos on the path between the rhinos and the Tsavo aviary and so on.
This would need some work and some expenditure, but I think it could enrich the experience of visitors by showing how the zoo has grown and developed through the years.
Alan