What do you mean by void? AN empty wall? Get some large art. Sites to look at: minted.com

etsy, greatbigcanvas.com

What are your interests? Do you hike, ride, surf, collect paperweights? A master bath is a very personal space, so make it yours. I would have photos of places I’ve been or waterfalls we have seen. If I collected anything, I would use professional quality photos of some of the things I love. You can take the pictures yourself or ask a friend who takes great pics. Find pictures that start and end your day with a smile or a lowered blood pressure. If you like this idea, be sure to use large pictures, three across or maybe a more formal two over two. Buy premade frames in several sizes (save the receipt ) and decide what size fits your wall. Then have the pictures made.

Art. Etsy.com is another source for art.

Figure out some way to hide the side view of the toilet. it's not attractive. Use a planter or maybe some type of ceiling mounted privacy screen.

This space needs either cabinetry with a seated area or a soaking tube.
I just don’t know how either of those would look.
Art work later.