Really like the addition of the dewlap!
Certain species of Mesozoic birds, such as Concornis, Elsornis, and Intiornis regularly perch on the backs of larger dinosaurs to groom them for parasites and to utilize the shed feathers they pluck out for nest material, not unlock oxpeckers do to the vast herds in Africa. Some could even take a parasitic route themselves, deliberately keeping wounds dealt by the player (maybe "the ones that got away" can reappear later in the game so that players might form 'personal' relationships with certain dinosaurs) in order to feast on the blood. From gameplay aspect, these birds could serve to make certain game more difficult to hunt, as they could act like lookouts or early warning systems to the dinosaurs they're perching on (and thus the player is trying to hunt).
Larger species of prehistoric bird, such as Gobipteryx (which are reportedly around the size of a pheasant) could be considered game as well, in addition to their larger relatives.
Considering how the big name dinosaurs won't be featured, but their ecological niches would realistically need to be filled, maybe featuring lesser known tyrannosaurids, such as Appalachiosaurus, and other carnivores could fill that need to hunt something that is not only bigger than you, but could (and would) actively hunt and kill you. To offset the killer aspect of the animals though, maybe Appalachiosaurus could be featured with a turkey-like waddle or snood (at least, on the males).
For the Zhenyuanlong, possibly feature eyespot-like patterns on the inside of their 'wings' that would, theoretically, serve to startle larger predators and give them a headstart, not unlike certain species of moths and butterflies.
Mesozoic mammaliforms, such as Castorocauda, Adelobasileus, and Xianshou could be trapped for their furs like how we trapped beavers, opossums, and squirrels for their pelts. Additionally, the player could choose to trap these and other smaller animals with stationary traps (bear trap, noose trap, etc.) in order to act as bait for larger game, which they would use ammunition on. After all, no hungry predator is realistically going to turn down a free meal they didn't have to do anything to get. In the case of the latter of the three, they would pose a challenge, considering their apparent ability to glide.
For Pterodaustro, in addition to essentially being the pterosaur equivalent of a flamingo (meaning they should obviously be pink, because that would be cool), the males could feature air sacs and grow display crests of pycnofibers on their heads during the breeding season, not unlike the greater prairie chicken; Like the chicken, these air sacs would be hidden beneath a thick mane of pycnofibers.
Wow you got some fantastic ideas here, great job! I really enjoyed reading these. Especially for the birds, protomammals and the Pterodaustro.
Regarding tyrannosaurids and dromaeosaurids, I'm leaning towards not including them in the roster altogether, or at the most including one of the smaller tyrannosaurs as a mid-tier predator. I feel that the overall shape of tyrannosaurs and dromaeosaurs is too familiar, and would rather focus on lesser known theropods instead- even if I do end up giving them outlandish feather arrangements and soft tissues.
In all honesty though, it's not all that hard - just looking at behaviors and features that present-day animals have. Plus, with your inclusion of features that wouldn't otherwise fossil well (or at all), like fleshy waddles, air sacs, and other display features, it means that you can include features that people otherwise don't associate with (real) dinosaurs. I mean, for all we know, maybe there were dinosaurs or other prehistoric animals that were actually capable of changing colors like chameleons or octopuses.
In regards to the tyrannosaurs and dromaeosaurs, I see your point. Just figured using more obscure members would be be just as sufficient, though I suppose unless it's a Yutyrannus or Tarbosaurus, anyone that sees a tyrannosaurid will immediately assume it's just T.rex. Still, I feel like their niche of large, apex predator would still need to be filled - maybe by carnotaurs or carcharodontosaurids? Like I said, you make any large theropod dinosaur and most people will assume it's T.rex simply because of how successful they were back in the day (and I don't just mean in media. If it wasn't south of the border, you can pretty much expect it to be a tyrannosaur of some kind).
I'm guessing that you aren't going to be all too concerned with dinosaurs from different periods existing at the same time, given how humans are involved in this project seemingly without a time machine? If that's the case, it might make things easier.
Yeah, I'm going for the hodgepodge approach. The setting is an alternate universe version of Earth, rather than our own prehistory, so it's a good excuse for me to go nuts and pick whatever animals I want, instead of worrying about whether these animals actually coexisted (in our timeline) or not. Going nuts is pretty much what this project is about, after all.
For the apex predator niche, I might pick one of the large carnosaurs, but we'll see. Still a long way to go, and there's a few animals lower on the food chain I want to design first.