A while ago there was a shader around, that could add fogging at a specific height - so you might have that at a certain height on your terrain, it worked a lot like standard fog.
But for a more organic effect, I would add in a particle system - get some fog textures and apply them to plains, then have the plains move and rotate very slowly - and fade out depending on distance to the player if you like. With lots of fog plains, you can move them any way you like, even have them react to collision so they stay outside. The main thing though is to use alpha transparent fog images - then when you have a bunch of them together, moving and rolling along, it looks superb.
I only did this once before, ages ago, with a rain effect (little splat textured plains positioned randomly, so they do look like splashes of rain). Anyhoo, when the varying transparency levels overlap, it gives a really organic effect - almost hypnotic in the eyes of the creator.
DBPro is a great platform for these sorts of games though, lots of shader effects, especially if you get DarkShader - and it's real easy to add in object clicking, like you can click on an object and find out what it is - ideal for a point and click adventure.
I would suggest that if you get DarkShader, let your artist guys deal with that - texturing and effects can get complex, and you could get very bogged down if you try and tackle it all yourself - better to have art people who know the 'rules', because that way you would just have to load the shader right in, and they can already have set it up how they want.
Health, Ammo, and bacon and eggs!
