Cel shader with color ramp generation and a preset save/load system to allow non-programmers and non-artists on small indie teams quickly create a stronger sense of atmosphere in their games.
Write shader with meaningful parameters for artists.
Create preset system with JSON so that users can toggle between different styles of the shader with one click.
Write custom material editor window to make it easier for users to get the look they want.
Create two variations of the shader that use multiple passes to draw outlines.