Yes, more “stuff” is definitly needed. The objects are fairly simple; rendering times should not be that high. I know the raytracer is neat and all but it dosn’t look like you have anything in your scene that is really benifiting from the raytraced lights. Try normal lights. Also the floor dosn’t need to be shiny and reflective; although I know it is hard to resist now
Everything above I agree with… plus the spray from the sparks and paint gun don’t seem very real at all. There aren’t enough sparks and the spray should be more misty than that. It does seem cool as a concept, but I do think this looks very empty.
The complaint about “too much space” does not necessarily mean “add more stuff”. It could just mean better composition. Put you camera nearer to the floor - about how high a person’s eyes would be if they were standing there viewing the scene. Then, move the camera around until you can get a nice view of everything that’s going on. Then, vary your lighting a bit to give some contrast - light areas, areas of shadow.
All of that will add visual interest to your scene without adding more “stuff”.