Indeed, a nice suggestion!
Of the already announced animals, peacocks might be one of the most obvious candidates for free roaming creatures. Personally, I really enjoy encounters with those "free spirits". (I live in Berlin and the "Pfaueninsel" is closeby and nice destination. Admittedly, it is not an actual zoo (they have some enclousers though), but I've seen free roaming animals in actual zoos as well.)
Or, not as an "exhibited animal", but part of every zoo in the world (I guess):
Wouldn't it be great to have "wilde visitors" in our zoos as well:
flocks of sparrows feeding on the other animal's food, hopping around in enclousers and freely finding their way to the best spots to be?