Im thinking like an micro chip implanted in our body. This system is not out of the picture because how we discover things every year, it is not impossible that in the future this things may happen and it will boom like btc.
I think it will be easier to pay with a fingerprint scan for example. Maybe we will have some database in a future with the information about the funds of each person and with a scan of your finger you will be able to pay like with a tap of a bank card.