very colorful

Thank you Shakúff, where can i find those?

You're welcome! You can contact us directly...They are made by one of our glass artists. http://www.shakuff.com/decor.htm

I would lean a oversized painting in one, and a oversized mirror in the other. Dont reinforce them on the wall, have them lean against the wall for a dramatic musum like effect

You could have natural sculpture something like this above the FP. It would be a hunt to find in nature!
https://www.houzz.com/photos/21st-century-bungalow-contemporary-living-room-new-york-phvw-vp~413007

Vibrating colors! Customized paintings to suit your decor is always look good.

I think working with a natural material in one of the niches - and art in the other, will be important for balance. I would use something wood in the stone surround. I've seen some great dimensional wood tile or round log slices with bark on that you adhesive to the back wall with one tacking nail for safety.
Then, for the other niche - art that reflects your color story - something large scale and portrait centered - right at the scale of it.

Thank you for the great ideas and suggestions!

I can custom create a painting to fill the space

I'd go for a copper or brass vintage metal scuplture. It would add dimension, the depth that you need, and warmth. We have many at groovywares.com

I think I would close one of them off ... either the one on the left with drywall, or the one on the right with a LARGE piece of art the covers the entire hole.

They look a bit odd there side by side, and takes away a bit from the fireplace as the focal point.

If you are a wildlife lover, we get many owl print orders - This one is a barred Owl. We can custom print our photos to fit. David Hemmings info@naturesphotoadventures.com