Thanks for your comment, indeed there are some work behind this ahah.
I have made an improvement (I guess sort of), unfortunately haven't see your comment before, but I have changed some things you pointed out too.
Just forgot about the bushes in foreground, I know they look like a dried lettuce or a bad flocking tree ahah.
I will make this test with a more simple procedural landscape. For me the main issue here, as always, it's scale: for some reasons, the white moutain top at 7000m, that why it's hard to put a convincing texture on it (lot of "empty" space to cover).
"Give the sky a more obvious gradient from light blue to darker blue, it looks a bit flat?" totaly right, I will test that.
"Terragen is a hard task master!" Clearly and that why I like it, you had to think in a radicaly different manner than most of other soft to understand it.