Here are some Cenozoic creatures I think have a better chance of being considered. Whether they exist in the lore, were mentioned, or are easier to create for the game using existing techniques they have for making models. First, the only Cenozoic creature shown in JP/JW lore alive, Smilodon. If a second Camp Cretaceous dlc was made, there might be a possibility we see Smilodon.
Siberian Mammoth: The bones were shown in an episode.
Since feathers are something Frontier does very well, a species of Terror Bird. Using the movement from the Gallimimus model, it shouldn't be that difficult to make one.
Having Aquatic Reptiles, it would be more doable to create a model of a Cenozoic marine creature using the movements from one of the marine reptiles. Some species that could fit.
Megladon
Basilosaurus