At the moment, given the current state of flight for large pterosaurs in JWE2, I'm not confident that aviary birds could be implemented well. At the same time I really want more habitat birds and think we could use a lot more to have a more realistic zoo simulation (as opposed to being so skewed towards mammalian megafauna), so I think the best solution would just be to add more habitat birds in upcoming packs plus a dedicated animal pack for them, e.g. a red-crowned crane in an East Asia Pack, a black swan in an Oceania pack, etc.
The possibility that's been discussed a bit, wouldn't be hard to implement, and would help a lot in enhancing realism is some kind of Children's Zoo expansion pack which includes a selection of domestic animals (donkey, goat, sheep, cow, pig, chicken, with maybe a couple that get two different breeds), an expansion to walk-in habitats that includes feeding and touching for species with the Domesticated tag, and barnyard props, as well as other things common in children-oriented areas of zoos--animatronic dinosaurs, simple rides (a carousel), a couple exhibit animals that are very popular in the exotic pet trade (leopard gecko/ball python), and an expansion of the educator system to include animal shows; maybe even an ambassador animal system (this could be the free update). Might not fulfill dreams but would be a massive step forward that might be more feasible than aquariums or aviaries.