There is nothing of which I know that will be sensible to use to lighten a wood ceiling in place. That means painting. Or installing something over the present material. There are a lot of materials that can be installed on ceilings. Tongue and groove wood/faux wood, tiles, all kinds of stuff. Do a little research before making up your minds.

My living room layout is very similar! I had my couch horizontal in front of your window creating a entryway, maybe put a console table behind the couch.

That looks pretty darn......................brown? If it's what you like, fine. But honestly.......how about some pillows? Lower the tv, and get an IR repeater. Invest in a console long enough to visually support the tv. Yes........to art. Perhaps to some shades with texture at the windows, versus the utilitarian and cold blinds? Your best bet, is to find some interior design help local to you.......and let her take you just a bit out of your "comfort zone" . : )

I've got one these right now. It's a beautiful space, but temperature regulation is an issue. Watching TV is an issue, too. The sound is lost in the room and we have to turn the volume up quite a bit to hear. As a retired appraiser, it makes more financial sense to floor the second story portion. While not a wow factor choice, it increases your useable square footage and will result in a corresponding increase in valuation. The point is, there are a lot more factors involved and unintended results of the decision to go with a 2-story high space.