This is good. Quality should be more in the final, if possible. Look at maybe adding a fluff of a cloud or two.
Lighting - have you tried moodflow or Seth's lighting ideas, where you change the Background shader into a Default Shader and play around with the Luminosity and the Environmental light settings? Another idea would be to take this Default Shader replacement and add an HDR image to it and maybe use a little translucency. If you do this, then your sun strength can be lower. Maybe put the sun on soft shadows, too.