I have a Microsoft Band and with the new Bandsider App there is a daily challenge, each day it is a random thing, it might be sprint to the end of your street and back or do 500 sit-ups etc.
I was thinking maybe Zwift could issue daily challenges based of a few different levels of ability and members could subscribe to the level that suits them. for example levels beginner, intermediate and expert. The challenges would therefore vary in difficulty and or length based on these but would change each day.