I'd go with either a large mirror with an interesting frame or a colorful piece of framed art.

Where is a good place to find such a mirror?

You could get one custom made at any framing store. Or go online to any number of furniture retailers. Just make sure you measure the space so you know what size will fit.

I might continue with the tone of the mantel/surround.

I think a beautiful piece of art would be the most interesting solution.

The space is great for a number of different things. A round mirror would be a good choice as it would be a change from all the horizontal lines in the room.

I would look through Houzz pictures and see what catches your eye. Use search terms like fireplace art or fireplace mirror.

Pier 1

Here is a link to the Houzz photo search page.

Fireplace mirror

Art would be best choice, unless you have something "interesting" to reflect.

Looks great. Pretty room. A painting with the same hues of the couch to tie it in

Art definitely. Big.

I’d go with something oval or round to balance all the straight lines in your shelving ....either art or mirror would work...just make sure the size is appropriate to the space.

There seems to be a disconnect from the beautiful fireplace surround in natural stone and the stark white bookcases. I’d start with that thought. Either change the color of bookcases to blend with the stone mantel or add more moldings above the mantel to connect to the bookcases.

following...

I just realized your tv placement. I would make an L with sofas then everybody can see tv.
Add some bold teal pillows; a tall plant on table and colorful art with teal , water maybe or floral?

A large painting, a Japanese jacket, or a rug would all work effectively.

Art for sure.

Art for sure, something with movement, not too busy, you have a lot with the shelving which looks very nice

A very color saturated abstract would be amazing! Love this artist on Etsy.com

oversized art..

A really nice piece of art with a pop of color will enhance this space tremendously

J Design Group
225 Malaga Ave
Coral Gables, FL 33134
Ph: 305-444-4611
https://www.JDesignGroup.com

How much work are you willing to put in? If it were my house I would do some sort of wood paneling like any of these photos...