Having coded quite a bit in my lifetime, I don’t think that it would be overly challenging. Time consuming of course, and requiring some testing, but not that challenging.
The reason I believe this, is that Darktable (from the surface of things) seems very well coded. The modularity and the way it runs, gives the impression of a very lean and well structured code base. That’s another reason I really enjoy using it. It can do so much, still it doesn’t feel bloated at all.
To me it would be very challenging. I’ve never coded anything like this, but to the devs of Darktable, I think it would be very doable.
As for sharpen-modules I notice this caveat about the sharpen module from the manual:
Note: The USM algorithm used in this module [the sharpening module] performs blurs in Lab color space, which can produce undesirable effects, and is no longer recommended. Instead use the presets offered by the contrast equalizer module for deblurring or the local contrast module for general sharpness.
I’ve used both local contrast and contrast equalizer, of which I prefer the contrast equalizer.
The reason that I tend to prefer the highpass method these days, is that I tend to get fiddly when using the contrast eq. Too much flexibility!
When using highpass I’m more direct to the task: if I think something is too sharp, I just decrease the opacity and/or the hardness of the mask. It gives a very quick and organic workflow for me. But it’s a preference that might very well change in the future – I keep learning new things, and the highpass method may have disadvantages that I’m not yet aware of. I could possibly have the very same experience with the local contrast module.
I’m not aware of any other methods of sharpening, besides the ones you listed. Maybe someone else will chime in?
I really miss a shortcut that will temporarily remove both the yellow mask preview, and the mask-widgets. I can press ‘W’ and get a full-size preview without these overlays, but it also changes the zoom of the preview, and it does not return to the viewpoint I had - I am zoomed in on something different when I return. Some shortcut that would simply just remove the overlays while pressed.