There is a feature I have not found in Apple Calendar and other 3rd party Calendars. It is a feature that most doctors require as well as anyone who has somebody else keep there calendar and make their appointments. That is, an accurate showing of Work or Office hours. Apple has a start day and end of day time, but this is not robust enough for appointment taking. It has to be detailed enough to show the time off for lunch, different working hours on different working days. Ability to project out into the future indefinitely, and easily adjust for holidays, vacations, sick days and emergencies.
Such working hours should be reflected via shading during any time hour ranges are shown (daily or weekly), and there should be API's for the calendar, such as "find next open working time of at least x duration".
This can be simulated by making repeating schedules of Work Hours as an event - but such a technique is confusing to look at, and clunky to adjust.
With this one set of features, millions of doctors and other professionals could ditch their paper appointment books and move onto Apple's Calendar.