Fill it in.

You could place a nice big mirror there. Custom build if need be. On the internet you can also find pictures in any size and scale you like. It's almost impossible not to find something nice.

If you do not like a picture or mirror that big, you could just fill the entire hole and even paint it a fun contrasting color, like black or white. And then on that you can place your picture or pictures or mirror in the size you do like.

If that doesn't work for you either, no one is forbidding you to get rid of the upper part or even the entire thing completely. It does not win the beauty price. You could try to put some fake stones on it, to make it look more real?

To me the warm glow of a fire is a wonderful thing. I can't have one and do miss one.

Oh, I see I commented as you were also adding some information.

You think filling that in is a hassle? One big piece of good board might do the trick already. It is very simple. You could also on that board put some fun wallpaper to highlight that area. Give it a bit of your time and effort. This can get real nice.

Maybe a mirrored cabinet will help. Something like this:

Mirrored TV Cabinet at Horchow

You could cut a piece of brick veneer that matches the firebox bricks to the size of the inside of the cutout and pop it in, leaving all the electrical components for future use by yourselves or the next homeowners. Then proceed to decorate with art, decor, etc.

That thing is clearly nothing but wallboard. It looks cheap and amateurish (and very '80s)

I'd get rid of it--it's easy. The downside? You'll have to paint the ceiling and re-cover the wall. It's an easy job and well worth it.

Or, just fill it in so that you have a flush surface and put a painting on it. With some color! All that gray is depressing.

Depends on how much money you want to spend. I couldn't live with that "thing". I would remove the whole mess and reconfigure the fireplace/stove with an appealing, safe approach. I would wonder just how safely that "thing" is built give the stove might be a retrofit? Worth checking if you haven't already.