What does this fireplace need? If I added a white wood mantle, would it look strange or finish it off? I am painting the trim white and the sliders will be replaced with white french doors. Thanks!!
A lot depends on personal taste. In my opinion, you do not need a mantle, and it would look wrong because of the huge visual presence imposed by the rocks that go all the way up to the ceiling. I also think that a mantle often does not look right when you have a raised hearth, because it goes up too high and looks out of place.
If I understand you correctly, you plan to paint the black metal trim around the firebox white. Several comments about that: How will the white trim blend with a black screen? The contrast might not look right. White trim will show soot and be harder to keep clean than black. You will need to use paint designed for use in a very hot environment. You should probably scrub the surfaces to be painted with steel wool so that they are slightly rough because the paint will come off if the surface is very smooth.
I think replacing the sliders with white french doors will look terrific. I would also think about hanging something on the stone wall above the firebox -- actually, well above the firebox, say no closer than 2 feet, so it does not pose a fire hazard. The kind of thing that I think will look nice and finish off the fireplace is a large something like a fancy patchwork quilt, a hand-woven carpet or blanket with a Southwestern design, or even something with a Scandinavian design with bright primary colors. I think something like that would really dress up that huge rock wall.
Good luck with your project.