This idea just popped into my head and I don't know if it would work or not...... Are you using PS? If so.... Filter/Render/Clouds on a new layer.....then mask out the cloud areas you don't want to show. That might make the background seem a little less flat? In all honesty I thought this image looked just fine until the flatness was pointed out....and then I could see what they meant.

I am, therefore I create.......
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