Zwift would allow to replay a race (ex. that I have raced in the past) with the ghosts instead of the real riders and myself. Riders could test themselves in races they have raced some time ago to see their progress or what the outcome of that race would have been with current fitness 
This is a brilliant request. It is also available in other training software. Not only could a rider re-race an event against them self and others. It would allow us to race club races that we missed. A feature like this would possibly reduce the overall number of races on the calendar and result in fewer schedule conflicts because we could simply choose a race from the “library”.