Hmm thats a hard one because it already has so much personality! not sure if its your style but a simple antler mount may look cool or maybe just keep it as simple as fresh flowers and let the mantle speak for itself!

Whatever it is should be no wider than the firebox opening. You might actually find a mirror that duplicates the arc of the detail there. Scale will the the kicker. Here's a shopping link on Houzz.

https://www.houzz.com/products/query/arch-mirror/nqrwns/p/36

Well-I like the spiders :)

As silly as this might sound, you’ll probably “know it when you see it.” And you might not be looking for it when you find it.

I’d measure the width of the firebox, make a note of it in my phone, and maybe see if there are some estate sales in your area, look at Etsy for some original art (sculptural or otherwise), and maybe check out any exhibitions the local community college may be hosting. Flea market and garage sale season is coming very soon too.

Relax, enjoy the hunt, and good luck with your quest to further personalize your lovely mantle!

From Allposters.com

It is an interesting fireplace with lots to look at. An empty picture frame or two, deconstructed look or an old window would keep it simple and the focus on the fireplace.

be careful of mirrors... only use them if they reflect something beautiful you want doubled in your space.

The fireplace surround is stunning. I would keep accessories simple. Then put more neutral rug in and add green pillows. Not sure how you use this room? But here is mock-up.

Items

Outside of a formal family portrait painting I think mantles change with the seasons so I wouldn't work overly hard looking for just one perfect piece.

I really like your eclectic style. Everything looks great.

Whenever I’m looking for something unusual, I go Chairish or similar sites with unique and vintage objects:

https://www.chairish.com/collection/objects-and-oddities

It might inspire or give you an idea.

I would seriously consider painting the fireplace to match the wall surround ..for a more modern dramatic ….but less detailed statement. With all that is going on in the room this will give a place to rest the eye while still embracing the drama…. Then find something sleek .. simple and black for the mantle to tie together with the black insert…

If you feel the need …a tall narrow simple vase with a few sticks of bamboo (painted black) on each side of the fireplace…would be in keeping with the striking contrast and simplicity

I love judianna's first photo. Mantels and hearths look so much better to me when kept simple.