I would not feel obliged to fill that space: the dark, textured stone is already a strong feature, while the TV occupies the quieter recess to the left. If the wall still feels bare when you sit on the sofa, I would try one wide, softly colored artwork centered over the mantel—something with an ivory or warm taupe ground and a little charcoal to connect it to the stone. I would skip a grid of small frames or a mirror, which could compete with the stone pattern and reflect the windows or TV.

Before buying or drilling, tape up a paper rectangle to judge the scale from your usual seats. If this is a working fireplace, check the unit’s heat-clearance guidance and the mounting method for that stone first. Leaving the stone bare and styling the mantel simply is also a good option.

Thank you

Perhaps a 3D object to contrast the stone texture.

Agree with roomxlab. The stone can be the accent as it has alot of texture.

how deep is the mantel? a single candlestick? or slim bottle/pottery?

The mantel is about 5” deep

With your modern room and the lovely stone, I’d add not too large art on the mantle and some pottery. Testing will really help you decide.