It would be quite easy to add a screen of some type behind the TV to hide electronics and make it look finished in the hallway. Lots of products out there. I think that is a better place for the TV than above the chimney and it might not be even safe above the chimney area. The pipe probably runs from the center of the fireplace up that right side of the build out. I would leave the area above the opening toward the ceiling open, but close the back of the area for the TV off, then you probably need to create a grommet in the top of that shelf to drop down cables and wires for the TV and perhaps some speaker wires? Not sure what you have. If you go to Home Depot or Lowe's you can go to their lumber area and see what panels could be used to close that off with something decorative.

It looks like there would be a better furniture layout but more views of the room would be needed to see what that is. Once that's finalized, an area rug would help to visually warm up the space. It would be easiest to purchase a rug before the art. As far as wall art such as paintings, I would definitely look for pieces with lighter colored backgrounds.

You have enough room for two three seat sofas and a couple of smaller scale arm chairs. Take a look at the below diagram to see if it's something that would appeal to you. It's got two eight foot sofas and a large area rug that defines the entire seating area. One sofa is served by cubed coffee tables, the other by a larger table with an ottoman underneath for casual extra seating. See below example. If the arrangement will work for you, get some blue painters tape and map out the furniture on the floor to make sure the sizing will fit.

Will these drawings help you imagine how your space could appear??? I'm suggesting you could assemble from IKEA pieces a desk/media unit for your TV wall.