Wow, I came back and found all your comments. Thank you.
Let's see if I can answer them all.
The background - the mountains mainly: I agree, it could be made more interesting, as I didn't do much for it, except for making the ground much darker, as it was too light - plus a "snow" and "grass" layer.
The snow: Limiting minimum slope is an interesting idea. I had a fractal, but it gave the snow a salt-and-pepper-look, so I gave it a coverage of 1, but still have maximum slope. The snow I have seen (and it's not a lot) was covering the "flow" routes much more than level ground - that's why I thought of World Machine. Except that this is a power fractal which is good for fields and planes. I did read somewhere in these forums that there is a way to save a TER from a power fractal, but it looked too involved. I will probably try that, just to import the flow map.
Fake stones: I had them and removed them, as all my texturing attempts resulted in their looking like almonds in the shell. Plus, I couldn't find a way to stop the "grass clumps" (the inbuilt ones) from growing on the stones. I have managed to "tame" other objects' distribution with masks, but the grass clump eludes me. The scene still has the very small fake stones - if you take away some of the grass, they can be seen.
Clouds: I like the cirrus and low level clouds better, as they are easier to look good. I found the mid level ones too noisy with lots of small wisps and did not use them much at first, so it is probably lack of practice. These use two density fractals merged in multiply mode (I tried subtracting, but it seemed to increase the cloud cover). The large scale one to control their general shape and the small one to give them details. I think the main problem lies with the large fractal, as it does not give them the shape I had in mind. The clip file I attach is another attempt with the same general idea. It needs more work.
Trees: I have no idea how to make the trees grow in bunches, except by using even more image masks or fractals and distribution shaders. As it is, my shader area of the nodes looks a mess (and should get worse, when I add more trees). Of course, that is not to say I have any objection to that, just that I am afraid I will reach a point, where the scene will crash Terragen, as I have 1 Gb of memory and the present scene (together with Windows), brings the total memory use to 1.2 Gb.
Also, in this image I used an old Terragen trick to increase the contrast and saturation: decreasing gamma and increasing sun strength, plus decreasing the bluesky saturation, because it becomes too intense.
Helen