Graphic wallcovering subtle or bold. Or decorative fretwork pre-fab panels like these from Wayfair. Come in pvc or wood.

What is that thing around base of toilet for? Curious. Your bathroom is a tranquil space, very nice.

I think it already is beautiful!

It’s a nice bathroom , how about a shelve with perfume bottles and towels and black bathroom mats . You can hang a painting ...but in art ..in a gray / beige setting ..you should introduce a colorful one .. or just stick with a few shelves ..perhaps a decorative chandelier , or a mirrored wall . What is that plastic base around the toilet ....? is it for children to keep their feet ? ..looks strange ...but if you need it ....., ..enjoy decorating ..it doesn’t take much ...

Embrace the tranquility! If you crave some oomph I would add some plants in natural planters since you have lots of natural light. Color can come from rugs, towels, and a item or two for the countertop, but I would keep the contrast minimal, personally.

Its a beautiful neutral backdrop with great lighting. You can use color to bring some life into it. It depends on your style, but carpets, artwork and towels can easily solve the issue you have. I think you need to add some warmth to all of those solid surfaces.

A large watercolor would be great in that spot - subtly colored, but colored nonetheless. What’s your favorite color? And be sure to Go Big or Go Home. Then add towels to match.

Are the walls gray? That contributes to the cold and naked look.