I think the ability to add a pin to a layer to keep it from moving in the layer stack would be a welcomed feature. I often use a reference layer at the top of the layer stack set to multiply at 50% as a guide for my work. I find myself having to move this layer back to the top often and a way to pin it would be ideal.
The addition of a small pin icon or the option in the layer panel flyout to lock the layer position in the stack would be a simple way to implement it.