Once a route that has been complete. An option to come up that allows you to do it again but in reverse for extra XP points.
Hi Jack, welcome to the forum!
Some routes are offered as reverse versions (e.g. Volcano and Volcano CCW), the other routes can´t be chosen as reverse.
Very soon you won´t know what to spend your millions of XP on anyways 
Its an excellent idea. When doing training rides in real-life I always alternate the direction I ride local routes, as otherwise is easy to slip into riding a route the way it is easiest.
Sadly though, I suspect somewhere at Zwift a programmers head is already hitting the table whilst muttering that its a lot more work than just adding a “Reverse Route” checkbox.