If Zwift were a playstation/xbox video game, it would definitely have a single player career mode. Start out as an amateur and work yourself up through the rankings. Go pro, learn how to be a domestique, and eventually have domestiques work for you.
Progress through the game would be based on a training plan - so you wouldn’t actually need to be putting out pro wattage to “win” single player mode. It would just be a fun story to follow that would be an entertaining option to the crowded course.