1. Aviaries and diving animations, so all kind of a birds and diving animals would be possible.
2. More activities for guests - all kind of kiosks/buildings/activities/touch pools/animals shows etc. ZT2 did excellent job with activities for guests while still remaining in zoo theming. Would also love to see more nature wonders other than animals that guests can see in your zoos - animal/dinosaur skeletons, minerals, rare plants, corals, annual rings of tree trunks...all cool things that you could make exhibits for. Many zoos do exhibit all kind of nature curriosities beside animals.
3. More interactions/animations for animals. Egg laying birds, nurturing animal mothers, animals interacting with glass barriers and with guests etc. Would be nice if each animal have more interactions with others in the pack/herd/flock instead of just one adult animation per species (and some even seems to have just the mating animation and nothing else, which is sometimes quite boring atm.). I know that would be a lot of work, but currently everytime you see your animals interact with another, it is the same animation again and again. If each species had like 3 non mating animations, it would be way more entertaining to look at them.