I would buy a much wider tv stand, your current one isn't in good proportion with the tv.

You will have room for decorative items on the tv stand and that will add interest to that wall as well. You can also fill the space with a large floor vase with a tall filler.

I'm new to architecture and architectural details and whatnot, but my first thought was to buy furniture to fill the walls (bookshelves, armoires, etc.), then I thought of picture rails.

What do you think of installing a picture rail (stained to match the crown and baseboards), maybe at the same height as the top of your window trim, all around the room, then you could hang art/photos/etc. from the rail?

Command hooks.

can you put the TV on the wall?