The first thing that comes to mind is to add wall on top of the existing so that the wall around the flue disappears. Would add a mantle and surround that is appropriate to the style of your house and your furnishings. Would also replace the hearth. Those brass inlays date the firebox itself. On some fireboxes those can be removed (almost peeled off) using a pair of needlenose pliers. Good luck!

Gorgeous space! Lots of potential. Looking forward to seeing what ideas come!

I also recommend adding wall to make it flesh. It might look great to do the white marble around the fireplace and all the way to the ceiling, then add a wood panel / mantle to create more dimension. 100% recommend taking off the brass.

Build the wall to the ceiling. Cover the black metal fireplace with marble slab. Add a stained wood mantel and solid brass fireplace doors. Drop in a decorative mirror and here's what you can end up with.

Of course, there are many variations of this look, but taking the wall to the ceiling will give you an open slate to create a variety of looks. Not sure I would use that particular mirror, but I was in a hurry and it was on my desktop.