Some of this kind of existed in the early days of Zwift. There were fewer real people using it, and the world was populated by “blue riders”:
So it’s probably the case that some of the code that could make this work already exists.
Your idea about “saving” a group ride would seem to line up with the feature request for “personal best bots”. I dare say Zwift would have to implement that before widening the system out into entire groups of ghost riders.
Do you not enjoy just joining the world and riding around with people in general?