An editor setting which could set the hight of fog would be nice.
Add an ASL value so you can set a desired hight of the fog layer.

E.g set fogASL to 50, and you will have fog 50 meter above the ocean and flatlands, but as soon you get some altitude, you will be able to see over the fog.

This would make for a much more atmospheric fog system.

Picture example: http://behance.vo.llnwd.net/profiles5/216345/projects/2933515/f433beb5a547875fed2996cd31e59e53.jpg