From my perspective, I just want to be able to chose from a few static cloud backgrouds, high cirrus, puffy cumulous, and maybe a couple lower sun angles, not sunsets, but to give some depth to the horizon.
I see - just for depth reference then, as a background. There are atmospheric lighting effects (Illumination/Show atmospheric effects checkbox in scene properties), do they help for the depth perception and horizon?
The atmospheric effect only works for global scene, and it's just a haze, and not adjustable. I am looking for a solution mainly for local scenes. Adjustable haze in the local scene would be step in the right direction, but what I'd prefer is a mixed cloud/sky effect above the horizon.
I used a layer mask effect in GIMP/Photoshop to replace the uniform background color with a static image of a sky. Pro still doesn't seems to have good solution to add an image file as a background instead of a color. Even if it's nothing more than an immovable background image and doesn't move with the scene, it's better than a basic solid color; and it saves an extra step of additional software for image exports.