Grease Pencil could have few options for Traditional Animators to use. Perhaps a new Timeline view which shows small thumbnails of the frames and easy to use handles for controlling the exposure of each frame. Traditional animators will mostly work with pen, and to minimize going back to the mouse, UI elements such as handles and sliders would be useful. These are implemented in other 2D traditional animation software such as OpenToonz and TVPaint.
Also, a quick toggle for layers to have a normal playback, a looped playback or hold on the last frame would be nice.
Maybe the Extrude, E could be used as the tool to create exposure for a frame? Right now it just moves the frame.
Also, GP is a vector animation software. So for raster painting, I would animate it on blender and then would like to export it to Krita and paint frames there.