If you just want decor, I would put a bigger piece of art over the fireplace and either two candlesticks (one on each side) or three (two on one side and one on the other). I agree with Patricia about removing the stacked tables. I think art on either side of the fireplace would be too cluttery.

Nice room. The fireplace/mantel is unusually tall, especially for a narrow room. No knickknacks on the mantel - instead, find a bigger piece of art to rest directly on the mantel. Art doesn’t have to be as wide as the fireplace, but bigger than what you have now.

Then add some other, smaller pieces that will coordinate with your new art. You wouldn’t have to put anything on the window-side if there’s room for a tall green plant in that corner.

A couple of ideas from above, plus three friends to live on the mantel -- the point being that anything you put there should be striking, of the right shape and essentially form one image. No bits and pieces.

I think it needs a surround, to give it definition and structure for hanging art. I don’t think it will be cohesive without one

Get yourself a proper mantel and surround and don't worry about the rest.