I have been thinking about the collections that I visited that are now closed, including Belle Vue, Glasgow, Flamingo Gardens (now private), Kilverstone, Leeds Castle, Mole Hall, Otter Trust (Earsham near Bungay), Peakirk WWT, Rode, RSCC and Winged World (Morecambe). All of them had some good features. Glasgow had an excellent site. Rode had splendid free-flying macaws and a fine collection of parrots. Kilverstone and RSCC had some interesting and unusual species, as has already been mentioned, but I don't think either of them displayed all the species particularly well. I think I enjoyed the Otter Trust most because it had such a beautiful site on the bank of the River Waveney and marvellous pens for the collection of otters (4 species if I recall correctly). It closed for the right reason too - there was no longer a need for its otter reintroduction programme: but that means that there would be no point in reopening it, unless there was a chance of establishing a collection of endangered otter species from around the world.
However the one I remember most vividly is Len Hill's original Birdland, on a tiny site in the middle of Bourton-on-the-Water which was not far from the current Birdland - so in a sense it has reopened already. The current collection has more space and much larger aviaries - but a much smaller collection and less charm and character I'm afraid.
Alan