I think every zoo needs polar animals
I am really hoping for a DLC with penguins, pinnipeds of some sort, and otters (of some sort).They haven't worked out proper swimming animations yet. You'll notice that the only animal that "dives" is the hippo, and that's because hippos don't actually swim, they sink and walk along the bottom of the water. The crocodiles and other swimming animals in-game only swim on the surface. I suspect making animals able to truly swim through the 3D environment like they did in Zoo Tycoon is something that takes a lot more work (I don't doubt they intended on including penguins in the base game, but I suspect animation constraints and an impending deadline saw them cut).
In any case, it's basically something they're going to have to provide at some point. Penguins and pinnipeds (seals, sea lions), and otters, are key zoo animals.