I would like to propose one interesting feature for Darktable. It is kind of combination of two already existing modules in DT in one instance (or separate one with more functionalities).
In DT there are ‘Color correction’ and ‘Color zones’ modules which allow to transform color in an image by hand, I mean by transforming handles in color palitras. In the module ‘Color correction’ it is two points in A/B color plane that change colorization of shadows and highlights. In module ‘Color zone’ there is possibilities to change color curves by moving handles of curve nodes.
Both of them has its own restrictions. For example, in ‘Color zones’ you need select one x-axis ‘select by’ for curves transformation and when you have done and decided select another principal axis for color changes all your previously constructed curves will gone away. As for ‘Color correction’, it affects only shadows and highlights.
What if in DT one can implement module which is kind of ‘Color transformation’ but with more functionalities when the A/B space can be transformed (sculpted) arbitrary. For example it was done in one small software with name ‘3D LUT creator’. It allows to transform the A/B color plane by moving nodes of a mesh drawn in the plane. This type of 2D color transformation directly changes colors of an image. For more clear pictures of what I am trying to explain I attached the screenshot from ‘3D LUT creator’. Anyone interested can download this piece of software and try it for free.