I think it would be great to issue a challenge to another rider while riding. It would be an ad hoc mini-race to a landmark in the game. I envision the Challenger pressing a “Race to” button for a particular rider (or maybe several riders). After pressing the button the Challenger would select an upcoming landmark such as a start/finish banner, sprint banner, top of climb, bridge, hairpin, Big Ben, etc… Selecting the landmark would issue the challenge. Then the Challengee would be given the option of accepting or rejecting the challenge.
If accepted, both riders would be set to identical routes to the selected landmark. Given the world crowdedness, it would probably be necessary to “pin” the time gaps for those participating in the challenge. This would ensure you always know where you stand.
I imagine you would also need a Do not Disturb setting to avoid users getting bombarded with unwanted challenges. Maybe some sort of indicator, depicting whether a rider is open to challenges.
It seems like these challenges could lead to new interesting stats tracking your mini-race performance. Things like wins, losses, best win (based on opponents FTP), worst loss. You could even have an ELO rating like in chess and other games.
I realized this kind of occurs informally, but unless you contact via chat, you can never tell whether that rider you were “racing” against was actually racing against you too.