@odysseywestra It is very interesting!
Especially about "Black & White / Colorize" of GMIC,
If that filter deeply integrated, I think it would be something another new tool, named "gmic-fill" or something like that.
Because, the user interface for that tool would be very different from flood-fill tool.
Ideally, I hope that tool can paint hint strokes with Mypaint brush, Interactively.