What do you think about making a covered board perhaps fabric or wall paper for the insert. It should be a simple pattern, organic color, close to stone color. It could possibly be padded.
It would cover the entire inside indentation which has a rounded top.
Then some price of art. Perhaps something metal. Large wooded candlesticks. A mixture of stone, wood and metal go nicely. Good luck

Beautiful room and stonework! We live in a subdivision built some years ago with similar tall ceilings and windows, plus the added bonus of those indented shapes above most of our fireplaces! Have seen dozens and dozens of my neighbors' homes with various attempts to decorate around those indents, some of us have tried pretending it's not there, but the absolute best solution, long term, I have seen is this: Have regular glass applied to the inside of the indented section, and then have a carpenter (or artist) install custom framing around the outside edges of the indent. It can even extend into and over the edge of your stonework around the mirrored section if that makes sense? Looks like a custom mirror hanging there. My neighbor had hers painted a distressed black with some subtle gold flourishes, that went with the fireplace below it and her décor. Then she uses top of mantel for seasonal displays that can change. Wish I had done that years ago! Hope that helps!!

A nice piece of sculpture would look beautiful to get the viewers attention. Find a piece that has a good contrast from the stones for it to stand out.

If you are thinking of going with artwork (not sure if you are though, I know you had mentioned a mirror or clock) but if you do feel free to take a look at my Etsy Shop :) I offer custom pieces as well.

https://www.etsy.com/shop/GPerillo?ref=si_shop

Thanks everyone for the suggestions!!

If you've got anymore, keep them coming :)

NICE STONE WORK. I THINK SOME BIG WROUGHT IRON PIECE WOULD LOOK GREAT AGAINST THAT FIREPLACE. GOOD LUCK

I really like Winkle's idea and think a piece of wrought iron whose lines mimic the shape of the indentation would look great. However, that option would definitely limit what could be on the mantel since objects on the mantel would visually intrude on the iron pattern.

Alternatively, a mirror mimicking the shape of the indentation with a rustic frame as misslaur suggested would allow more flexibility on what you could use on the mantel.