OK, I know the game is all about being in the right spot to cap a tag, but when at a gathering or talking to new people about the game, it would be awesome to have a tag I could keep on my key ring that could be capped anywhere(and have the locations recorded). Even better if it could be my Gold Star. Maybe that sort of item could be in the store, or maybe assigned to a username so that each player only had one. At a gathering, players could get points for meeting other players.
If you want them to be limited, then the Gold Star or "one per profile" would work great, but if you want it to be a revenue stream, then selling special codes would be best. Those codes could also be used in other games that have traveling items...I think you know what I mean.
I guess there would have to be some way to avoid abuse, like players making lists of codes.