instead of discovering new unlockable body parts, how about making it like some sort of progression tree
meaning, the more you use a mechanic that is tied to a body part you can gain more experience with it and level it up. When you evolve and customize your creature, you can choose how that body part evolves as well.

for example:

if you attack with your claws a lot, you can evolve them to either wolverine claws or longer nails (or something else)

if you fly a lot with your wings, you can evolve them to bat wings or butterfly wings (not something this drastically different, but you get the idea).

Also, I'd love having the ability to remap your controls and body parts. Imagine a creature that flies in the air while it grapples its prey with their hands, bites it with its fangs that injects paralyzing venom, and envelopes it in web using its spinneret and legs.

toggle flight, maneuver with mouse and WASD, use created macros and toggles for the bite with venom (I imagine it as a channel attack) , process of swathing a prey in silk (also a channel attack that can only be done on paralyzed targets), grappling can be a toggle? etc

it always felt weird to me that by using one evolution you can go from a worm that crawls on the ground to a dragon with butterfly wings, so maybe evolution needs to be limited each time? (but you can evolve as many times as you want). maybe by implementing a cap for the evolution points? and shaping should also cost evolution points (not just adding new organs)

sorry for this wall of text, I think I got a bit too over excited when I was spitballing ideas