A little magazine rack and better storage for your toilet rolls. Maybe another picture on the long wall. Since it's the guest toilet, you may be better off saving your money to put into more utilized spaces of the house.

Replace mirror and lighting. Consider some staggered floating shelves styled with art and decor.

Paint the walls something other than white.

The mirror is a bit heavy to me. Maybe an oval mirror that is on a tilt with some cylinder sconces on either side. Go bold with your artwork or, as Jillian suggested, add some shelves with colorful glass and decorative objects.