I built out this concept of a "could forest" within Octane Render inside Cinema 4D. I used a VDB Volume to develop the main cloud, and then within that placed geometry to use as the base for my forest. The plugin Forester for Cinema 4D allowed me to create a lush and dense forest with a variety of trees. I was able to mix and match and also get an ideal distribution that I was looking for among the trees.
I composited the 3D over an actual photo of clouds, it felt more natural to see real clouds and their detail than relying on 3D to do all the work. Nature elements can be very tough to develop entirely in 3D so I felt like a blend between real and CG was a way to make the sene slightly more believable looking.
The mountains and waterfall is just some landscape primitives arranged in a way I was happy with, then I create a particle stream in X-Particles cascading down the side of the mountain, and just used one still for the mesh. I added some VDB mist at the top and bottom of the waterfall.