1st remove those silly looking sconces. . Do you have an art college near you/ If so get one of the students that you like their work to come and do a painting in that space or if there is someting worth reflecting on the opposite wall then have a mirror made to fill that space. Add some larger taller sconces if you have to have them

Wow, that is a pretty mantel. The sconces are placed too low, I believe, and seem small for the overall look. I think you could go with a painting that is square-ish, similar to what you have there now but larger. Look for something that is about the same size as the black fireplace box. Just my opinion, but I wouldn't put a mirror there. I know there are lovely mirror frames out there, but mirrors don't say anything about the personality of the person living there.

Well said d_gw. I would install taller sconces and a round mirror.

The mirror and sconce mock ups look great!

I was going to suggest finding sconces with up look to replace the small ones. As to a mirror there, it would work, IF, there is something pretty to reflect? Is there a pretty chandelier? If not art would be best. You don't have to worry about the arched top. Just put blue painter tape about 4-5" from the straight sides and measure to see what size would work best. What furniture do you have in the space? Modern, traditional, etc. Do a frame on the art that works with your furniture styling.

I love this mock up you did! To be able to see it with the pieces is perfect! Thank you so much

I'm guessing, but probably the size will be around 36" wide by 24" high edge to edge. With a 3" frame probably art around 20x30 or so. A floating type frame takes up only about an inch, so just depends on the art you like.

If the previous pics are too stark, here's a more collected approach...

With so many angles and curves, I would do what Patricia advised and go with a mirror or mural to fit that frame.

http://www.susanharter.com/blog/2016/12/7/design-dilemmas-you-can-solve-with-murals

I looked on Etsy and there are shops that will customize wallpaper and murals to size.

These are a larger than yours, but the concepts will work for you.

I don't know the size of your space above the mantel, but you could also buy a large iinexpensive canvas and cut it to fit.

https://www.wayfair.com/decor-pillows/sb1/green-wall-art-c215363-a11291~41556.html

This might be a little unconventional but some lattice and a maintained grape vine could be really neat

I also would remove the sconces. I think they ruin the lines of the fireplace.

This is someone's take on an English overmantel. Have a mirror company come and do a template and fill that space with a mirror that is custom fitted to the space.

The sconces are all wrong. They're way too small and they're put ridiculously low. Get the mirror first and then see what works for sconces and hand them significantly higher.

Who doesn't love a faux animal head??

If you're bold like me, you could paint the background a darker color (black would be my choice)

I agree about removing the vases as well. Your mantel has some exquisite woodwork. No need to gild the lily. I'd probably hang a single piece of art.

Keep it simple. I like to add some greenery and layer it with art. See below for some inspiration photos. Good luck!