For sure artwork in a theme that you love. I'd also paint the registers the same color as your wall.

What about pieces of furniture?

Ethan Allen has nice quality furniture. The Arcata sofa might work. You can change the color on their site.

I just bought the new sofa and chairs

It looks like you are going to do something to the face of the fireplace. It would help to accessorize it.

Maybe a floor lamp on that back wall and a new lamp. Still with the artwork.

A table on that back wall might work also.

I’m out of my league suggesting anything with regard to the fireplace, art, etc, but for sure I think you need some plants. You’ll be amazed at the difference just a few living things can have on a room. Even the brownest of thumbs can grow a snake plant, and peace lilies are also very easy.