The room looks very nice. I'm not sure why you think you need to put something on that wall.

Nothing.

A potted plant or tree

Should I at least hang a piece of art?

I might hang art if the sconce was not there. The sconce throws it all off, IMO.

If it was gone, you could do art with an art light above or below it.

The floor mirror doesn't belong in its location, or for that matter, not even in the room. Try it is a bedroom or hallway. Then put the picture light over the mantel art.

The white planters will work well with your decor.

Yes you can try artwork on that wall. Please - something with a bit of color in it.