I know your sectional is new but is too big and dark for the small room, but don't panic, I have some suggestions that will help. Add four or five light colored small patterned throw pillows, replace the matching ottoman with a coffee table, hang your TV on the wall, remove the sideboard and store your AV equipment in the kitchen cabinets and use a radio frequency remote control.

I agree that the sofa is too large for the space, but S&K has some great ideas...for art you want something without too much density to you do not take up too much more space....a simple metal wall sculpture would do the trick.

Drape or shades over the window, with a piece of horizontal art to 'fill' up the wall space on the left side of the photo.

Have you considered adding trim around the window? That would give definition to it. As far as the walls, I have seen nice long shelves with taller vases and twigs flanking each side. I will see if I can try to find the picture I am referring to.