Here are a few things that I wished were features while making a custom model for a friend:
Arm Splay: a setting that causes the characters arms to naturally rest farther from the center of the body. This would be useful for characters with large dresses, or large lower halves (such as wide hips or big armor).
Accessory layers: An option to hide or unhide an object mesh when wearing certain accessories. For example, your character might have large hair that gets in the way of most hats. You could hide the big hair mesh and underneath would be a smoother hair that would be revealed when wearing a hat. This would also be useful for other accessories, such as necklaces, glasses, bracelets, backpacks, etc.
Still don’t think it will happen for Workshop player models, but I am going to keep repeating it until the day the devs DO decide to do it anyway - Custom clothing for default and Workshop player models especially. I remember someone in the past bringing up an issue on how this would work, and to that I say: Just make the thing somehow adjust itself to the model, I’m sure some math could be done for that. This could also work great with accessory layers that you are suggesting! An example: A model has a shirt that you consider ugly on a personal level - You would like the model to wear a different shirt, but not have to spam the Workshop with 2 to 3 versions of same model that only differ in clothing they wear. I swear, we NEED this.