Definately get rid of that mirror. Then you'll have to see what's behind it. If there is sheetrock, you can just paint the room. If not you'll have to put up sheetrock, then paint. Don't know what colors you like, but because of your flooring I'd stay away from any wall color that has a yellow tint - it will clash. The floors appear to be in good shape from your pic. You have a very large room, so a sectional sofa would be great in here or even two sofas facing each other (one on either side of the fireplace with a large ottoman in the middle. Mount a TV over the fireplace. Curtains that hit the floor on each window with the rods at the ceiling height of the side windows and match that height on the other windows. Place all furniture on a large patterned area rug.

Do you have any furnishings you plan to use or are you buying all new? First I would look to remodel the fireplace with a style that extends up to the vault instead of the present mirror which only reflects the ceiling - no view or anything desirable. While doing that I would put in serious windows, French doors or eliminate flanking windows in favor of massive build-in around the FP on that end of the room. Are all the windows in the room equally small and off the floor? What is the home style and your decorating style? Lots of questions need to be answered for really good ideas to blossom. If you have your furnishings, put them in the room and re-post so we know what you're working with. It is a great volume of space and deserves some input to become all you hope it can be.