I am not sure I would put anything over your mantel....Maybe that mirror you have on the floor or a piece of artwork. Make the picture/mirror part of the fireplace grouping....maybe hang a couple of inches higher than the top of the mantel. Anything taller would look out of place. I would like to see a tallish plant on the window side of the fireplace and on the other side of the fireplace, a basket, or a low stool or even a low busy plant.

You begin by drawing the room, and using a measuring tape, you add the dimensions. You post it here. You include the size of hearth, windows, and especially the opening to kitchen and hall area. Nobody can see around a corner here. You ask , am I going to have a tv? How many folks will sit, What will they sit on, and where will I put that seating? What size will that seating be? It isn't going to look good, or be passable to foot traffic, with much more than a sofa, and two comfortable chairs. Take the sectional word out of your head, from the looks of the photo. You ask yourself, how do I want the room to feel- what's my style? And the first thing you consider, BEFORE running to any store, is unifying the hardwood and eliminating the edge to edge carpet in the space. Then you consider losing chocolate brown walls : )

I dig the banners Groveraxle posted, they're 8ft high! That's a statement worth making. I advise against hanging most art, like your own examples, so far above eye level. Just ignore the wall above and concentrate on finding items that are a nice proportion to the length of the wall/area. We humans tend to concentrate on what's in our line of sight. Too high or low, eh. For the most part, art should hang at a level that's easy to appreciate the work. The center is usually about 60" from the floor. Otherwise, it's hung in close relationship to piece of furniture, at a comfortable view level. So go lower when viewing it from a seated position. Beth showed examples of maximizing the horizontal plane. That's what I would recommend, as well.

I think your entrance could become much more welcoming, so before buying a rug, maybe look at inspiration pictures to select the right style vs just the right size. Based on the walls and the black chandelier, modern farmhouse could be a great look. Don’t have to do everything at once, but if you have a plan, it will be coordinated as you make updates. Would place a very large rectangle rug, placed after the staircase, so it makes that wider area more prominent and highlighted by the decor Suggest moving your table and art to the upstairs landing and now you can coordinate the whole space using a longer console with art or mirror on the long wall. If you add baskets on floor it will help hide the heating element. Where your previous table was, add a bench and hooks, both being really handy You could paint the interior door black if owner allows On upper wall of staircase a set of large family prints would be very nice