What other colors are in your house, in the rooms adjacent to this room?

I'd avoid wallpaper just because the room feels like near perfection and wallpaper would distract from the great sense of layout and balance.

These random colors. and jumped to mind and certainly are not everyone's cup of tea. But, a small powder room is a chance to create a separate little world that is not connected to the rest of the home, so ave fun.

I like it just the way it is. Put a piece of art over the toilet and you're done. I like color, but the neutrals are really working here.

The lightest one that Kendrah posted would give the room some depth without ruining the balance you’ve achieved. I’m a fan of wallpaper but agree you should keep it elegant, and simple, as is. Very nice!

That wall is too small and full for wallpaper unless you get rid of something that's already there.