I think dinosaurs are unnecessary due to JWE. Frontier already has their dinosaur zoo game, so hopefully they won't put that one high on the priority list. I wouldn't be opposed to a pack with animals that humans killed off, including the dodo and thylacine, but it should be at the bottom of the list. As others have pointed out, we're still missing a ton of conventional zoo animals (again, as others have pointed out, the African rhinos - but also the oft-mentioned meerkats, otters, and a huge variety of primate species, not to mention the lack of birds). Crytpids I think would be a silly thing to make a whole DLC about, but that's just my opinion. If they made a little "animal only" pack that was cheap with the yeti and unicorn in it then fine, but I think over-all it would be a niche market and wouldn't earn them much in terms of money. Plus there's no precedent - Zoo Tycoon included the bigfoot, yeti, unicorn, and Loch Ness Monster, but they were never intended to be a focal point of the game. They added dinosaurs due to the (at the time) popularity of Jurassic Park and Walking With... and the sequel followed it up at the end due to the high demand when it was clear the franchise was coming to a close (hence why Extinct Animals was the biggest expansion for Zoo Tycoon 2).
Planet Zoo is still young. They have plenty of time to develop DLCs, but I don't think it's unreasonable to expect them to focus more on the animals already "missing" from the game before they deep-dive into niche markets.