Have a contractor add studs, drywall, and close it off. Find a tile you like and tile the entire thing. It's asymmetry......Lose the shade on the window above it. It;s awful looking. If you must cover it? Find art you can customize to the size. Put a nice large leafy potted plant to the newly dry walled and tiled side and it will be fine.

Make sure you soon meet code...........and removing tile may NOT have been the best idea. Plan on replacing with heat proof SOMETHING. Don't use it until you do.

I would add a few shelves painted the same as the wall color. Add books and some nice decor items. Not a bunch of small things, but a few larger items. Maybe add lighting for some soft illumination of an art glass piece.

Simplest affordable ideas:

--floating shelves for the niche

--Attractive bamboo shades for windows as below

Cape Cod · More Info

--Stained glass piece over window or, as suggested, a wide horizontal art work.

Extra Large 24x60 Abstract Landscape Acrylic Wall Hangings by Thomas John · More Info

Stained glass:

Etsy: this came up when I put in "vintage stained glass window"

--another Shoji paper lamp in niche. These are also from Etsy. Try different search terms.

Agree with shelves and curated items on the shelves - a super simple approach would be to stack them with nice looking wood

Obviously spending the $$$ to center the fireplaces solves it once and for all! That said I would frame in a box the same size as the fireplace on the right and fill it with wood. It will at least look intentional. Or put a tv there with logs burning on a loop :)

Not sure those white blinds add to your space. Here are some options

Beverly’s idea to stack wood on the right makes the asymmetry look deliberate. The whole area is lovely, but as Jan asked, is it fireproof? Start shopping for tiles if not.

Drywalling the window is an easy fix, but you lose that light source. A simple inside mounted blind, shade or anything else not made of fabric can help disguise it. Using the same material on all three openings makes sense.

IMO the drywall idea and tiling the whole wall makes the most sense if moving the FP is not an option BTW symmetry is overrated but balance is not .

Cozy up, add lamp and........no need for a window you obliterate. Clip in some painted plywood and find some great panoramic art.

Easy peasy and custom size

https://www.etsy.com/listing/691101714/tv-cover-landscape-tv-cover-pull-downs?external=1&rec_type=ad&ref=pla_similar_listing_top-4&frs=1&plkey=70332003b7a3e68fbda986b45f716af552d1218d%3A691101714

I’d add plants. They’re beautiful and purify the air.