I have two suggestions: 1. a wall mural or 2. a large mirror. Either would work, just depends on your taste. A large wall mirror would visually expand the space, and mirrors that face each other, as I believe they would in your bathroom, can be very interesting to look at as the reflections bounce off of each other. You could go for a framed mirror, or one of those beautiful Venetian style mirrors that have become so affordable lately. These are all mirror but create a "frame" from mirrors with etched designs in them. Many are very ornate but there are some nice, simple ones available. Look for something rectangular that is at least 36" (900mm) wide.

A wall mural makes an entirely different statement. The most important thing is to have a talented artist do the work so it doesn't look amateurish. But then the sky's the limit. You can have a faux window painted with a "view", any kind of scene, or something as simple as flowers and vines. I have attached a photo of a Tuscan scene as this was the owner's favourite place in the world. The other two photos are of a stylized flowers and vines for just the same type of space, a blank wall over an open tub. The motif was carried around the room with subtle bits painted in all corners.