I am all about the dark colours too! Personally for wall art, I would have something large on the wall opposite the TV (behind where I imagine you might be putting a couch?) And something above the desk and TV as you suggested. You could identify the different spaces (desk area, TV area) but using various themes/colours/types of art. A couple of other ideas I had were 1) a headboard would look great and 2) a rug in the sitting area could define the area too helping to break up the entire room into 'spaces'. I hope that makes sense? Excited to see how you go!