You could use an external plugin to render a demo effect called "plasma" to create the fog.
An easier way would be to use a rendered plasma loop of two or more plasmas interfering in intensity and animate it's overall alpha within a script, so no loop looks like the other.
Well, while thinking of this method:
Use TWO plasma loops, with different lengths, at best the frame numbers should be multiples of prime numbers, and stack them in the scene editor. If the alpha is set correctly on both loops, they will interfere with each other, creating the fog. Since both loops have different lengths, they will never create the same interference twice until restarted. You can use scripts to animate both alphas and/or the starting frame of every loop to create further randomness.
