I think you can do all of this already:
1) the cloud color and scattering color give you control over the cloud colors. However, often you don't get the colors you;'ve chosen since it is heavily affected by density, edge sharpness, lighting and the cloudfractal.
2) Like you can restrict surfaces to height you can do this also with cloudfractals. You should be able to do this using distribution- and merge-shaders to combine the cloudfractals.
3) Would be nice indeed

4) To add to your suggestions: more fractal noise flavours

In summary, clouds are a b*tch

You can get very nice results quite easily, but when you want to get very realistic results the learning curve becomes very steep quickly.
Martin