There's a lot going on in bathrooms, in the room, behind the walls and under the flooring. The smallest room requires the most thought because of the importance of maintaining privacy (throughout your decades) while enabling the space and fixtures to accomplish why you're in there in the first place.
(This post describes a master bathroom but most of the considerations also apply to half-baths and powder rooms.)
There's so much to plan and position in tight quarters, and you might not realize what's behind the walls from other, adjacent rooms, like ductwork. You must consider function, form and style within scarce footage allowing proper spacing for approach, turning and use of the toilet, vanity, shower and/or tub.
This isn't launching the space shuttle but demands forethought of personal hygiene needs and preferences over multiple life stages and the potential circumstances of the individual occupants (e.g. children, elderly, pregnant wife, injured athlete, etc.).
So in order to have a "bathroom for a lifetime", let's start by listing some general must-haves:
*A huge money saver now versus later, don't let anyone tell you this is expensive or difficult. Quite the opposite, ever seen how much gets pitched on a construction site? Bracing with extra wood for future grab bars or a ceiling mounted, transfer lift is smart planning. If you're ever in an accident or need for whatever reason, the walls are already prepared and installation will be a snap.
Next I describe the importance of an accessible and comfortable throne.