One way to implement this great idea is to copy what Gran Turismo (GT) on the Playstation does for gamers. Zwift is aware of the Gran Turismo user controls since Zwift Academy is based on GT Academy. GT allows any user to configure a room where the competition rules are set, including level requirements, courses, duration, # of racers, experience level etc. The room can be by invite or open to all. The room leader decides when to close off the entries and then the race starts on a private track. The equivalent of a GT room is Zwift event.
Rooms are accessed by a Lobby, which gives the listing of races with the full description. With tens of thousands of GT users online at any time, there are hundreds of rooms to join. A menu system makes it simple to configure a room, open it up for entries and start the event. Leaders have the ability to punt someone from the room at any time. Zwift can copy the ability to configure a private event on the fly, then when the Event leader decides to close the room, the event will start. In GT, racers will advertise their events on message boards, or the event can be by invite only among friends.
Zwift would need to automate the ability to create an event on the fly and build it into the user interface. While Sony and Playstation are much larger than Zwift, there is no reason that this could not be done on a limited basis.