Currently, Model support only works for simple mobs that have one part for each "limb". As an example, a zombie, which has a single left and right arm part, a single left and right leg part, and a head part. For more complex animals and monsters however, it stops being functional. As an example: Creating a bird with a beak part that needs to move with the head.
Solution: Allow us to bind multiple model parts to the same limb/thing and bind them so they move the same. So when the bird's head turns, the beak stays in the same position relative to the part it is bound to (the head). Currently this is technically doable if someone digs into the code and writes it out themselves but in my experience this has been really buggy and having to dive into code means I cant go back and edit anything with the simpler editing tools.