Prehistroic Kingdom has no living animals. Meaning that people are still unable to truly complete things. Neither game has both caribou and mammoths. Both ground sloths and anteaters. Both bluebucks and springboks. This is what I want, and I believe that Planet Zoo can do it best.Mentioned this before (and others as well), people who are looking for this - they should really check out Prehistoric Kingdom (currently in Alpha)
Among with dinos, mammals have been confirmed as well like Daeodon, Mammoth, Hyeanadon, Megatherium , Megaloceros, Woolly Rhino, Antique Bison, Smilodon, Dire bear & Paraceratherium.
(Geekism has couple of vids on gameplay - looks really good)
I've put this game in my wishlist as well, some weeks ago.. I think Frontier won't be competing with other devs when it comes to such features.. They will never get close to the detail of PK.. They actually focus their game on this.
Maybe if you advocate for more mammals from a specific period in a game that's currently in Alpha - you'd actually have a real chance.. It fits the theme perfectly.
Also, several zoo games have added distinctly "non-zoo" animals. It's far from a requirement.