Hi there! Assuming that is brick on that wall, the obvious thing to do is paint it. I would paint it the same color as the adjacent woodwork. It will appear slightly different in color since it is a different surface, but that is a good thing! It will add interest to the wall.

I think I would paint it the color that is between the lightest beige and the darker tan but. lighter than the fireplace surround and mantle. I would like some contrast between the mantle and the built in cabinets. Not matching cabinets and not white. Just painting the fireplace wall.

I would paint the walls above the cabinets, the same color as the cabinets. The fireplace will become the focal point of the room.

I would remove the mirror above the TV. Leave the wall bare.
Use the two guitars as art on a wall of their own. Mounting with easy access to remove when needed.

If the floor lamp is not next to a chair or sofa, I would change it's location.

I would also paint the brick - everything but the hearth in front of the firebox. I'd favor white. And I agree that the mirror should be removed. It's too high and distracting there.

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