Provide ability to “ink” a line drawing. Provide a means to color fill bounded areas - similar to the way color fill is handled in Procreate as an example. After using the pen tool to define the lines of an illustration - like for example a cartoon character - it would be very helpful to be able to colorize the line drawing by dragging color in to bounded (leak proof) areas created by these overlapping lines.
Sounds quite similar to the existing Live Paint tools in Illustrator. Have you tried it?