What material should the mirror be made out of. Wood frame rectangle or circle. So many options and so much already going on here.

A great way to lighten the look of bookcase would be to line backs of them with some lighter wallpaper or fabric. An easy way to go this is cutting foam board to fit, covering it and then attaching with command strips.
The two top bookshelves look good but bottom two look a bit crowded. Try moving the 3 blue books horizontal and then add object or photo on them. It’s hard to see what’s all with dragon, but I think you could remove a few items the re-assess.
https://www.houzz.com/photos/highland-park-family-room-transitional-living-room-dallas-phvw-vp~6594252

Try turning your mirror vertical. I personally don’t like leaning mirrors, seem like an accident waiting to happen and then they just reflect ceiling. Bottom of mirror shouldn’t be any more than about 5” above mantle. Keep additional mantel decor, if desired, to scale with mirror, meaning not to small. Something green might be nice to play off floors, and green goes well with tan leather.
Is there another place for the red artwork? It’s cool but looks out of place there.

You have some photos in your ideabook, which is a good start. I realize the photo below is not your style, but showing you how painting the bookshelf the same color as the mantle will be a game-changer.

Right now, the eye is going to the bookshelf and it should be going to the fireplace and bookshelf together. Please do the following

1) Remove the mirror, small stuff on mantle and red art. Add a piece of art that is 2/3 -3/34 width of mantle and less than the height of the bookcase.

2) Decorate bookshelves simply and with a common theme.

Need photos of all 4 walls of the room to make more recommendations, such as arranging furniture correctly.

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No mirror all it is reflecting is nothing worth seeing twice . IMO the elephant in the space is the TV niche with all that stuff above the TV. I would close in the top shelves and just leave the TV that is dawrfing the FP and fighting it totally. It also would bea big help to see the whole space but if this was my room that built in would be ripped out and that wall drywalled over and the TV hung on the wall .

Ok thank you to everyone for all responses. I did try changing the mirror vertical. Fits better. This is a work in progress. New job, new home, new roomie/boyfriend. The bookcase was there when he moved in. It covers a large awkward inset area that is next to fireplace. See pic. I suggested moving shelves down and mounting tv on wall. Ken feels it would still look awkward. L shaped Couch is placed in front of tv area and fireplace is to the the left. Couch fits perfectly in corner and extends to end of wall behind it so not sure how it would fit any other way. Wall behind couch shares wall with stairs.

So, if you can take that shelf down, that would be a winner. Furniture feels large and bulky for the space.

Yeah it was moved from a bigger house. What would you suggest? Needs to accommodate seating for family get togethers and parties.

And take all the black shelves out completely and hang tv on wall? There would still be a cut out space in the wall from ceiling to floor next to fire place

Options:

1) paint bookcase and fp same color

2) remove top shelves and hang tv. Maybe a couple of floating shelves over tv.

3) drywall it in and then no cabinet under tv.

I would remove that large chair. Need more photos of the room to comment more.

My impression is that there are 3 ok vertical rectangles: fp/mirror; bookcase which should have chalk paint to similar medium tone; window shade.

Mirror seems fine, and nice details - metal? tile? Bookcase is for color, right in the middle of the 3... ?

Take the decor items off the mantle. They are too small to be effective.

Switch the square art near the window, with the mirror. The square art is squished in that space and will look, I think, great over the fireplace. The mirror near the window will reflect both the view outside, and reflect light into the room. Remove the long rectangular art to the left of the book shelf. Too many contrasting rectangles everywhere in this room, including the blinds. You need to break that up.

I see 2 round pieces of wall art, one on the wall near the dining table, and one on the floor. Can we see them in better close up? It's possible that the round one would be even better than the square art, over the fireplace.

Is the fireplace wood or?

Cut some cardboard to fit the back of the shelves, and paint it (or cover it with paper) approximately the colour of the walls. Hang the circular wall art you already have.

Add more things-that-are-not-rectangles. Move the sculpture out of the corner. Put the plant in the corner, raised up on a box.