It depends how you want to use the space. Two tall bookshelves would be great if you have books and objects to style with. If you entertain frequently, a bar cart on one side with a low bookcase on the other would be fun! You can add art or mirrors above. See below for inspiration images. Good luck!

That's really a nice room. So much to love about it, including the windows we can't see on the other walls. I agree with colourhappy's point about negative space, and also with those who feel for other reasons that nothing is needed in the high spaces. The first and third photos posted by Rebeccamom123 illustrate keeping attention down in the levels people occupy. But those rooms benefit from a unified upper line defined by the architecture that you currently don't have. Rebeccamom123's middle photo shows wonderfully that that isn't the only way to a beautiful space, but this design is far more complex and required some talent. And I think yours also requires a little talent and training to make it look good. That TV placement may be necessary, but to me it creates a really significant visual problem. Since you're adding a mantle, etc., perhaps consider hiring a good designer to pull it all together? For instance, just because the TV support and lines come out at that point doesn't mean the TV itself couldn't be moved down (and sideways?) to a better position, adding architectural detail to disguise as needed. (Btw, strictly in the interest of feedback, one of those corner side walls looks big enough for your TV, or next size down, and I'd almost certainly have put it there. If it needs to be on the end wall to be seen from the dining and/or kitchen areas, and a TV in the kitchen's out of the question, of course that's the place.)

I love your current furniture placement. If you are buying new, I'd try a sofa by the windows, and two side chairs (swivel would be nice!) where your current sofa is. Put a coffee table in front of the sofa, and a sofa table behind it (with lamps), and then add a table between the two chairs.

No decorating there. It would be helpful to see the rest of the room, rather than just that one corner. So, just from that one photo you posted, I would hang the TV on that wall. Perhaps get a TV that is a bit larger (they have become very affordable in the larger sizes), and hang it on the wall. You can first put blue painters tape on that wall to see what size looks good. I recommend a wall mount bracket by Peerless. Peerless makes good ones, and even articulating ones if you need that.