I've not posted on the PZ2 threads all that much as I'm still working out what I'd like to see beyond flying avians, marine animals, a scaling tool for building and more modular/customizable exhibits. I have a list I'm working on of animals I'd like to see but it's a work in progress and has about 350 animals I'm trying to separate into categories.
I really do like the idea of slowing down the passage of time. Something like 5 speeds total instead of 3 - with the middle one being what we have now. Imagine having time slowed down to a fourth of what it is now, but all the animations and actions are still the same speed, you just get to know your animals better that way because they live longer. I agree with what's been said, as cool as it is to get new animals there's a disconnect from the animals themselves. Maybe not when you first start a zoo out, but because breeding in general is so easy, that this part of the game mostly just plays itself in the background. I cannot tell you how many inbreeding and birth impending notifications I get because they pop up constantly. Death impending I may or may not spend time on, depends on the species.
The other thing I've really wanted is more personality to our animals. It kind of goes with the above, there's a disconnection from the animals themselves aside from the excitement about having the species in the game period. I'd love to see different personality factors of the animals, much like we have with genetics. Are the individual animals high or low energy (do they play a lot or sleep a lot?) do they interact with others of the species (or humans at the barriers) or are they loners? Are they more passive or assertive (or for a challenge even some that are aggressive) when it comes to those interactions? If they interact with others, would the animals be able to form unique bonds with certain members of their family? I think trying to have a species hierarchy is sort of the beginning of this, but they could do a lot more with it.
Another big thing for me is having interactions between species, not just a bonus for having them share habitats. Aside from the predator/prey system, the animals don't really interact with other species, which I realize is pretty common in real life as well. But there's definitely something to explore here as interspecies interactions do happen, it's just not the norm. Again talking outside of a predator/prey system, that's definitely the normal part of the circle of life.
All of these things would work to make our animals even more unique, not just on a species level but on an individual one.