This thread is a good read, with suggestions from the sublime to the ridiculous. To suggest Chester should have a white tiger is, well, rather missing the point of Chester Zoo, and I really don't think there is a need for cutting and pasting of exhibits around the zoo.
As some have suggested, there are some things that can be done, quickly and relatively cheaply. Ditch the current marketing; replace it with something more tasteful. Bring the visitor into a quality experience rather than a tacky one. Make sure the breadth and depth of the collection are brought to visitors' attention with thorough sineage, maps and a guidebook. This can be done without taking the fun out of everything. Quality, and entertaining a family, are not mutually exclusive.
Keep on building as they always have, keep growing and evolving, but make quality the watchword. And remember the spectacle is in the animals on show. They are what visitors come to see above all else, so make it as comfortable as possible for them to do so.