fossil osprey
One of the biggest incentives for tax-ing animals in COTW is seeing their trophy in your lodge and appreciating their appearance. Animals with antlers and horns provide that satisfaction -- but animals such as predators have absolutely no visible difference between a silver vs a diamond. The only way you can tell apart predators is their fur color...
Having most/every species with a skull or weight trophy organ vary in size depending on difficulty level would be very satisfactory. Scaling the model and making minor adjustments to things like the torso/tail/head to make the anatomy remain accurate is all it would take.
I'm not a developer, so including size variations in even one species model could be much more complex than it would seem to an outsider -- but if its possible without any huge roadblocks in development, I would love to see it one day.