Yeah, the fog looks good there, but the trees are a little flat. And going back to what you were saying about rocks; I think it would be cool then if you added a little moisture to the scene elsewhere, maybe a puddle or something. Wetness always makes things more claustrophobic and dark. I think a little color in the back could be cool too, a really dark, albeit saturated green. It looks great though, don't get me wrong d:

And yeah, Maya Fluids can really chew up render time. Another thing that could be easier to add some depth to the scene is a light mask, to give subtle chances in light intensity. You could just drag a procedural cloud map onto the light color, but reduce the thresh levels on the cloud to avoid it being too extreme.

Going back to the puddle, you could achieve that just by adding a flat plane that comes over low areas in the scene, make it really reflective, with a little transparency. Maybe make a few random meshes and give them a dark black texture, and distribute them underneath the plane to add some particulate to the water.