Outline Modifier for Grease Pencil

we need an Outline Modifier to draw an outline to whole GP object or selected layers...

this modifier would:

• Updates itself if layers or strokes are animated.
• Got an option for offset value.
• Be accessible to other modifiers too, like thickness or multiple strokes
• Assign material, or at least got stroke material options.
• Because GP is in a 3D world and it could be drawn in different depth, outline must be defined in a 2d work plane perpendicular to X , Y or Z axes or a custom vector.