Consider putting some elements of brighter areas inside the mansion (lights? highlights?) so as to draw the eye inside the design of the house, not merely to the silhouette of the house. The curvature of the horizon is better than a straight line, but it's still quite nice and round. Think about adding a little more "torment" in the topography of the land so it doesn't look like a nicely rounded surface. Watch out for tangents (the tip of the roof is close to the illustration's edge; the tree is very close to the edge of the house). The strong cast shadow of the house is too dark because the clouds would reflect and diffuse the ambient light more. The sky looks very attractive (great motion in it!). Anxious to see the final version!!