If more windows is not an option, consider adding a skylight.
A skylight not only lets in daylight, but can be opened to increase ventilation.
However, if the bathroom is on the first floor of a multi-story house, a skylight may not be practical. Light tubes are small, very reflective tubular skylights that can be installed where traditional skylights do not fit.
A 14" tube, can be up to 15' long (to reach even a 2-1/2 story roof) and can snake around obstructions.
It tipped out in the laundry room to unload the clothes into the washer. A lot of old houses had them, and we don't understand why they fell out of favor.
The drawer has a cut-out that fits around the sink and compartments to hold all the things you use at the sink — brushes, pins, Q-tips, hair bands and so on.
If the vanity is being custom made, the drawer are a simple addition.
The space in the wall between studs is ideal shallow storage that can run from floor to ceiling.
How about a floor to ceiling medicine cabinet built right into the wall? Wall cabinets are typical in kitchens for convenient storage, rare in bathrooms. bathroom-style wall cabinets on both sides of the sink and above the toilet provide very useful and convenient storage for linens and all sorts of personal items.
We seldom see a bay, bow or box window in a bathroom (unless we put it in), yet these are very effective not only in fostering the illusion of more space, but actually providing more space — a wonderful space, indeed, for humidity- and sunshine-loving plants. Curtains and shutters undo all the good effects of windows.