Here's what I would do. Move the coat rack left of the front door. Push the credenza a little farther toward the stairs. Hang the mirror vertically 5-7" above the credenza. This way it will provide a visual counterbalance to the big vent underneath, rather than a duplicate of its horizontal shape. Consider a rug about 2 feet longer than the credenza that won't interfere with the door swing if centered with the credenza. Don't put any credenza legs on it.