Wow, twenty-foot ceilings!
If the walls are in great shape, I don't think I'd do anything beyond paint. I'd do something to the ceiling to "bring it down" visually. Googling "decorate bathroom high ceiling" I'm seeing people adding wood ceilings that match vanities, adding horizontal trim part way up that hides recessed lighting, adding skylights and chandeliers, putting in shelves way overhead that hold decorative items.
What I'm not seeing, at all, is wallpaper.
If this were my bathroom, I'd get some kind of molding and put it in somewhere between 12 and 15 feet from the floor. I'd install lights hidden by it from below (but accessible with a ladder) and probably paint two tones, one above the molding, one below.