pretty room! I could see a collection of three brass candlesticks on a corner for some height and then I'd probably want a trio of pale (blue?) pottery - different shapes and sizes would add some interest, etsy is great for this -

Love the suggestions from la_la Girl and Emily. Definitely consider a frame for the art. Keep mantel decor simple with nothing below the art. Adding some blue would be nice.

Beautiful elegant room you made here!
The soft champagne gold is lovely and the room is soft. You do seem to be missing a little depth however. I would suggest adding a plant next to the couch and while you did not ask for decorations around the room other than the mantle, I think a little decor on the coffee table or an end table/pouf would look nice.

If you only want mantel decor, agree with the rest here to add a thick frame ( preferably in warm wood or walnut wood)

Plants and a couple decor pieces on the mantle will look very nice.

IMO your furnishing positions could benefit from some tweaking. See photo adjusted below.

The corner near the window could use a tree and keep things to a MINIMUM on the mantle

I would switch the candlesticks in the photo with the vase but something light blue or even chinoserie could go well. I would also suggest you to move the end table away from fireplace and put it inbetween the two armchairs. If you want to go past the mantle, Blue or mixed blue and gold pillows on each armchair will add some depth. But on its own the mantle having a simple  vase and plant or fern will look stunning.

I have no new advice but wanted to chime in on how beautiful and serene your room is!

Agree the room is very nice…soft and calming. The blue vase from SF Interiors is lovely with the art. I also like the trees from SF Interiors and Beverly.

a wider frame in gold for the artwork. a slim candlestick or two or some other tall gold decorative piece on the left side of mantle (facing it), and a small piece of greenery on other side, a thin vertical plant on wall next to sofa in a large round pot or a tall decorative piece of sculpture, A round blue vase or decorative dish on coffee table. you can add a complementary blue pillow or another contrasting blue throw on chair next to fireplace. The room is serene snd calming. you do not need to add much else to what you already have