I find myself avoiding shorter routes on a course because I either have to repeat the same route on a ride or try to navigate using turns (admittedly not a strength) to switch things up. Easy for me to say, but it seems like the ability to pick several pre-planned routes to ride in order would be a relatively simple addition.
Make sure you vote up the existing suggestions at that similar to yours so Zwift knows what the users want: https://support.zwift.com/hc/en-us/search?utf8=%E2%9C%93&query=Custom+routes&commit=Search
https://support.zwift.com/hc/en-us/search?utf8=%E2%9C%93&query=pre-planned+route&commit=Search