Yes for the 'shiny action lines', both around his head and earth; it will give it a nice sense of motion and add to it, especially since his hair kind of blends in with the background. Also, the left side (our left) of the ribbon the medal hangs from looks a tad flat in comparison to the other side, which follows his clavicle. The earth seems a bit flat as well, but that aside, I am worshipping you. I <3 this pic so much, and it's turning out gloriously!
Cell shading yes, but make the colors slightly off, to accentuate them a little more. Also, I think definately halo of action lines, on either his head or the world, though probably his head, unless you make the lines yellow, then the earth.
I think cell shading would match the best with the image as it currently is. And if you do action lines, maybe around his head? Dunno, if you do more planets and stars it might end up too busy for action lines.