Scale is critical...artwork, large mirror, fill the space.

Can you post more pix of the whole wall? And the whole room? My first inclination would be to drywall over it but that depends on the rest of the room.

Here u go

Thanks for the pic. Yes, I would definitely drywall over that niche. Even though I get the theme of arches, your windows are nicer to look at. Do you need blinds or curtains on them for privacy or light control? The skylights add more busyness (no, not saying they’re bad), so keep it pared down. Perhaps you can replace the mantle with something a little more substantial once the niche is gone. I am not a designer so I can’t really offer decorative suggestions. Me personally, I like simple and uncluttered. I have a mirror and 2 family candle sticks that are too small but were used for Christmas.