I'd say, drop the sun even further to about '0' to '1' degrees elevation and increase its strength to around '8'.
The scene is really dark just now, nothing grabs your eye so, that's how I'd accentuate it. The clouds would really take a nice hue of the atmosphere decay colour whilst keeping the near ground in sillouhette. Your sun is above those cloud-forms in the distance just now, I think it'd look much nicer with them being lit from beneath by the sun.
I'd probably lose the 1/2 sky to terrain ratio, aswell. the terrain is really uninteresting, I mean it is black, half the image is in effect empty space, nothing in it at all. Tilting your camera up so you have around 1/3 balance of sky/ground would add more interest to the viewer's eye. Make the focus the clouds and sky, not the ground.
EDIT: Image and edited file with the above adjustments. ^^ This is the same render settings you had except that I raised the AA to 3 from 2.
I reduced the density of the cumulus layer by half and dropped edge sharpness to '1' then slid both cloud layer quality sliders all the way up. Also, raised atmosphere sample quality to 45 from 16.
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It could still be doing with something, a point of interest.
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