When we eventually get aviaries I think the door will be opened to smaller mammals. It's a careful balance, though, because some small mammals are perfectly acceptable as habitat animals (such as tamarins, marmosets, meerkats, and so on) due to the amount of space they need in a zoo. Others, like chinchillas, or indeed hedgehogs, tenrecs, and so on, are extremely difficult to contain in open-air enclosures due to the variety of niches they occupy (climbing, digging, etc.) and their size, which makes them easy to lose.
Then there are those that might work in either - platypus, sloths, and of course some ground birds.