But fungibility IS limited while you are not at home.   No buying a hotdog from the street corner vendor that doesn't have the approved limited-fungibility-bitcoin terminal, or the girl scout selling girl scout cookies until after you get back home and convert it.

There is really no need to have such a system until bitcoin becomes the only form of payment.  In spite of anarcho-libertarian wishing for this in the near future, it's not gonna happen for a long, long, time.  VISA/Mastercard's legacy system would be sufficient, but I'm sure they would rather have your money under their control (as a demand deposit or credit) as it would be currently than sharing partial control with them.