pretty much disregard its assymetry. or even embrace it. I'd have fun with it, like leaning bigger vertical art where the height allows, and a smaller one next to it, to the left, maybe even interlapping a bit. You can put taller plant there too,,,does it get enough sunlight? Some plants would do great everywhere, so even if it doesn't you'll have some choices,

if the width of the mantel is enough you can even put a couple books on it. In the part that's lower. An interesting candle(I have candles everywhere in case the power goes out. lol). Something you collect, grouped together. doesn't have to be many pieces. Just a few you love. It's a nice fireplace.

I like asymmetrical arrangements on fireplaces such as yours. This isn't as wide as your fireplace, but you get the idea - a larger piece on the left and something, perhaps stacked like these, on the right.

I would say a taller urn of taller switches...