Oh, boy, you're gonna go down the rabbit hole with me! I made like, an eight page document on my dream zoo simulator game, ha ha. (
here, if you're interested)
I always wanted to see a greater variety of exhibit types. Instead of just fence and tank walls, how about aviaries? Aviaries would open up flying animals as an option. Sea pens would be kind of neat. How about acrylic tunnels that allow guests to walk through exhibits, including aquatic ones? I always thought it would be nice to have the ability to create small exhibits, and buildings for them, too. Like you could design an aquarium building and design a bunch of small aquariums for it. And there could be a special gate/door that allows guests to enter exhibits and walk around. Great for an aviary full of lorikeets, but not recommended for the tiger exhibit! Also, biome would be more flexible. Rather than having "rainforest" and "desert" and "tundra" paint tools, you'd have different types of terrain, and fresh and salt water. And maybe certain animals, like peafowl, could have a free-roaming mode.
Customization in general would be increased, like, you could design the logo of your place and it would appear on souvenirs and signs. When you place food and souvenir stands, water fountains, other stuff like that, there would be more themes to choose from. For retail stands, the theme can change the merchandise that gets sold. (like, an Africa drink stand might sell lion souvenir bottles while the Ocean one will have bottlenose dolphins) When you create buildings, you could choose different types of walls and flooring and staircases.
And of course, more animals! A modern zoo game with a decent budget should be able to offer a lot of animals even just as the base. If there can be over 700 Pokemon, I'm sure 100 zoo animals wouldn't be a big problem, right? That allows for all of the "stock" zoo animals along with a lot of obscure species. Expansion packs and DLC could allow for nearly endless options that players can pick and choose. Plus, any DLC packs that cost money could have a portion of their proceeds go to conservation. Speaking of DLC, it would also be cool if the game developers teamed up with a real life zoo or aquarium to release a DLC pack. Say they did it with the Monterey Bay Aquarium, you download it and now you can add structures and animals found at MBA.
More complex gameplay would be nice too. Doesn't have to get crazy difficult or anything, but just some stuff to add new challenge. One aspect I made right away was an accreditation mechanic. If your zoo meets the requirements, it can get accredited, which comes with pros and cons. There's also an animal personality feature that affects the needs of the animal, and can also affect whether or not they're suitable for things like shows and interaction programs. (an orca attacking a trainer or a sea lion attacking a guest make for very bad PR!) Breeding would also be a bit more complicated, you could choose to put animals on birth control or sterilize them completely. Genetic traits, including color morphs, could be passed down. Inbreeding animals could cause negative traits to appear. Hybrid animals such as ligers and wholphins could be bred. In game, there would be other facilities you could contact to do breeding loans and AI with.
Obtaining animals would also be a little different. Rather than just buying animals outright in one marketplace, there would be different animals available at different times, from different sources. (and some sources are nicer than others) Rather than all members of one species costing the same, the cost can vary depending on the age, source, personality, training, and reproductive status of an animal. Animals can be obtained through purchase, trade, and rescue. Zoos with a high rating have more options (both with species and sources) to choose from. (though in Sandbox mode, you can just pick any species and choose the personality)
Some kind of online functionality would be nice. Like getting to enter guest mode to walk through a zoo your friend makes, or uploading zoos that other people can download and share. The biggest online feature would be the ability to create a "network" of zoos. When you create a network, you choose which types of zoo lots are available and you choose how much money new players start with. There could even be the option to set limits, like "extinct animals only" or "rescue animals only", if you want to create a themed network. You can chat with other players who have zoos in the network and work together to do things like loans, trades, and breeding. (though I guess you wouldn't be able to receive animals if it's on an EP or DLC you don't have)