Maybe the blank spots are fine and I’m just not used to having things be open. 🤷🏼‍♀️

Try showing the two adjacent walls. Do you prefer to put out a lot of seasonal things that you change every few months? If so, perhaps a couple of long cloth or paper Halloween banners could be added. Some people prefer having empty space around a focal point.

The eye always seeks a place to rest, so I wouldn't worry about it.

I would take down the shelf and try the painting only on the wall - not centered on the wall, but instead of the end sofa cushion instead. Lower it a bit, too. Then, place a fun piece of pottery on the small table ... the pottery, little lamp, and picture will form a bit of a place for the eye to travel to....

I would remove the shelf, too. Is there any way to rearrange the room so the sofa is centered on the wall? It all looks squished and off center right now. If you post pics of the rest of the room, people will help you try a better arrangement.

What is that piece (throw? quilt?) folded on the back of the chair? Is it pretty enough to hang on the wall?

Large doesn't necessarily mean expensive. Textiles, like a small (baby) quilt or rug, can be less expensive than art.

Most of my seasonal decorating involves garlands--black leaves with purple for Halloween, pine needle garland for Christmas. They can bulk up a vignette.