Though I'd recommend FreeCAD over OpenSCAD as a 3D modeling tool for most users as an alternative to traditional CAD tools...and therein lies the distro problem.
Gimp is really not great for photo editing IMO - it really shines at photo manipulation. (i.e. it is Photoshop, not Lightroom.)
For RAW development (a la Lightroom), DarkTable and RawTherapee exist. I've only tried the former, and found it incredibly difficult to use.
I also prefer Krita for digital illustration (though maybe that's unfair - it's best at digital painting specifically.) Of course, if you need vector support, Inkscape is the obvious answer.
Blender for 3D graphics.
Gimp for photo editing.
Inkscape for illustration.
Though I'd recommend FreeCAD over OpenSCAD as a 3D modeling tool for most users as an alternative to traditional CAD tools...and therein lies the distro problem.