I personally wouldn't mind extinct animals being added in DLC at all, even if they were Mesozoic dinosaurs. Keep in mind, the dinosaurs in JWE are all designed to be consistent with the movies' 1990s-influenced aesthetic. That's why the raptors and related species don't have feathers, all the theropods have pronated wrists, etc. JWE is a game set in the JP/W cinematic universe first and foremost. If anything, a dinosaur DLC for Planet Zoo would give Frontier the opportunity to model dinosaurs with greater accuracy, since they wouldn't be shackled to the JP/W canon like with JWE.
That said, the likelihood of Frontier making a dinosaur DLC may not be so high, especially if Planet Zoo releases with more polished and engaging gameplay than JWE did back in 2018. I don't know whether the people who licensed the JP/W IP to Frontier would appreciate it if the customers were to flock away from JWE towards a Planet Zoo DLC with more accurate dinosaurs. It would limit the appeal of JWE to the most hardcore JP/W fans.
On the other hand, prehistoric mammals in a DLC wouldn't present this same problem. Many of the popular "Ice Age" species, such as woolly mammoths and their neighbors on the Eurasian and North American steppes and tundra, probably wouldn't appreciate the hot and humid climate of the JP/W isles anyway.