I was thinking that having an “opacity” option for the layers would be great. I usually use a “rough” layer under my “inked” layer; and I put the rough at 20% opacity or so.
This could be implemented like in After Effects :
Layers could get some properties when it is “maximized” : this would be a nice structure to put in place (kind of a stack), which would allow to be extended with other properties later ! Eventually, those properties could be animated with keyframes…