How about a one on one battle mode: ride around as normal but with an indicator on any rider that is open for a mini race. As you ride by, you can invite the other for a battle, they accept and Zwift creates a finish line at a random point down the course. First one over the line gets points / rewards etc.
Could be refined with riders defining preferences toward hills vs flats, or short vs long races and rivals matched on this as well as ballpark FTP. Could even start to create league tables based on wins.