The ability to mark a calendar as archived (i.e. retired). Archiving a calendar would remove it from sight when you are in the default calendar view, however accessible through an archive tab. That way old calendars can be removed from sight, but not deleted.
This is similar to how boards can be archived.
Possible Use Cases
If a calendar is made for a specific team or project, when said project is over or said team is disbanded, instead of deleting the calendar, the ability to archive it would remove it without deleting it. Doing so would make the calendar list less clumsy (less filled with unused calendars).
Another use case is for people who create separate calendars for each year. Being able to archive old unused calendars would clean up the list of calendars to a list of active calendars.
For example, I like to create a work, personal, and school calendar for each year. Overtime my calendar list is filled and cluttered with unused old calendars.