Can you give me ideas for decorating this space in my bathroom?

Do you just want to put something on the wall above the toilet? You could do 2 rows of long skinny photo ledges like these ones from Ikea and arrange framed pictures and plants on them. https://www.ikea.com/us/en/catalog/products/20446236/

Here a pictures of two of my bathrooms. I really like metal pieces in the bath. In the first two pics, All the pieces are metal. Some came with the finish they have, others I spray painted to coordinate with the other finishes. The frames were either metal picture frames or had mirrors in them. I removed these and spray painted them. I bought small wood items like the birds and twig, spray painted them and used a dab of hot glue to attach them to the wall and then hung the frame on the wall. The big black thing is of course a TV that I can watch while I take a bath or put on my makeup. Love having a TV in the bath. The metal basket has incense sticks in it that I think are pretty and they smell good.

You need either shelves or some kind of picture on the wall.

There are lots of bathroom makeover ideas here that you can get inspiration from:

https://www.hometalk.com/search/posts?filter=Bathroom%20in%20Make%20Over

You could try to paint this wall and make it a 'accent piece that stands out with its color,without overwhelming the rest of the room.I also just saw a great makeover where they installed wood paneling on the wall behind their toilet.Looked very classy too.Perhaps you can,as others have suggested on here,hang a pretty shelf up,that can double as storage for toiletpaper,tampons etc.You can hide those 'gross" products onside textile or homemade drawers.