In short: animals that don't belong in the zoo
- Domestic animals, which kept as house pets. Domestic is too broad and some species would be a nice addition to the game. I don't want to see "pets" in a zoo. (also very very unlikely)
I did see a children's petting zoo once that had various farm animals displayed, including one pen that had a very bored looking border collie. I am guessing (and hoping) the border collie might have been an employee's personal dog they were bringing to work with them, and the collie got pressed into service. There was a display describing how these dogs worked livestock.
As for animals I would rather not see, I don't think cetaceans belong in captivity, at least not for the purpose of entertaining humans (temporary captivity for rehabilitation of injuries is different).
I'm okay with very rare animals that are nearly extinct in the wild being brought into zoos for the purpose of breeding and saving the species. The San Diego Zoo Wild Animal Park (now called the Safari Park) did a lot of work with Prezwalski Horses, California Condors, certain rhino species, and Arabian Oryxes when these animals were extinct, or all but extinct, in the wild, and they (and other zoos) have done much to bolster their numbers and reintroduce them to their native habitats.
Maybe a game dynamic in franchise and challenge could be introduced where a zoo needs a certain reputation or education/conservation rating before they are "allowed" by the game to adopt certain nearly extinct species?
I agree with the poster who said they are not all that excited about adding more insects and spiders (except maybe butterflies for a butterfly garden exhibit or something), though I've been to interesting "bug zoos." I'd rather see more variety of snakes, lizards and more rare amphibians (some urodeles, such as hellbenders, axolotls and so on, would be cool), as well as small mammals and small birds as exhibit animals, along with the option for different sizes and shapes of completely enclosed exhibits and a reprogramming to increase guest interest and to increase zookeeper efficiency in caring for exhibit animals. Real life zoos keep their anacondas and Burmese pythons in much larger exhibit enclosures than they do smaller species. As the game is now, I think exhibit species are a wonderful feature but underdeveloped.