The obvious option would be framed art, but there are other things, like ... collections of decorative plates, decorative baskets, hats, group of embroidery frames with fabric stretched over them. If you 'do' Joanna Gaines, there are always big signs. Basically, if it's fairly flat, it's possible.

Then again, you could also hang a mobile from the ceiling there which would provide movement while also visually 'filling' that empty wall.

First, i love your banister!!

I have a similar space and I’ve been pondering it for a long time. I think I’m going to go with a ceramic installation. The ones I like are in the thousands, but I have faith that I’ll find something. I like how it could create movement, be abstract or something like birds or flowers (I’m thinking abstract). I found some great examples on Etsy.