So, maybe say "welcome back"? Great render anyway, not only in size. I see you're still working on it, so here's my thought; maybe use displacement intersection to get some old smooth snowfields (but perhaps that's not possible with an imported heightfield). Also, I would vary the greens and let them gradually fade into the snowy areas, perhaps change its hues in higher areas as well to a less lush green. If you use a loose altitide mask (no final altitudes), you can warp that as well, and have a less regular vegetation line.