The possibility to add an atmosphere.
For example a dusty room (exaggerated picture:)
(http://www.diydoctor.org.uk/project_images/floorsanding/Sanding_dust.jpg)
With a slider for intensity
add alpha maps...insert a plane and place it in front of you camera...add a texture with alpha channel
thanks for the idea, was already working with multiple planes.
I'm using transparent dirt pictures (png). makes the scene a bit light though.