Hi matt,
It’s nice that you have opened the scene to see the canyons. But you have to keep an eye on the ff:
Perspective -you have to nail this first to avoid further tweakings of every elements.
Light source
Realism - the very aim for every shots
Function of elements - artistic licensing is okay. But as much as possible elements should have sense. And/or should contribute something into the image.
Distribution of details from fg to background - some matte paintings tend to have inconsistent level of details and we should avoid that. Look for high res stocks and as much as possible avoid painting manually, since realism involves realistic details which can’t easily be achieved through painting.
Colors and values (due to the presence of atmosphere)
And since you have opened the scene, you’ll need a lot of time and effort to make every area of this image realistic and convincing. I also have opened my scene and it’s hard. I really intend to make it hard since i love testing my limits just for fun. And i find this kind of approach in my entry more interesting (for my own view). But of course we need to follow instructions as much as possible. Goodluck and hope that helps 