First up, the biggest thing that I see is missing is all the detail in the ground material. You need to get some rock normal maps and put them on all the surfaces. Make sure they make the ground look rough and as detailed like real rocks.
You could also go with some adaptive subsurface if normal mapping is not enough. Adaptive subsurface would make it look really nice without making the render times painfully slow (closer details get higher resolution geometry, distant details get lower resolution geometry)
The next thing would be to start filling up the scene with extra details. Real mountains and cliffs have many more elements. I suggest adding lots of smaller rocks scattered everywhere, shrubs and plants in all sorts of crevices, etc.
The other thing I noticed is that the world doesn’t seem as bright as it should be, especially with what is direct sunlight and scattered clouds. There should be much more contrast between the lit areas and the shadows.