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.
It is already a reality. The system name is proximity sensor payment system. Check the below video to know more,
https://www.youtube.com/watch?v=hTBJ6OIGkzcHowever, till the time the sensor is installed according to your own wish and no government controls are in place, we are all good with it. However, if this is enforced by the government, then you will be tracked for your every movement, every single transaction and every single thing you do in your daily lives. There is a difference between enforced and intentional. So it should be intentional rather than enforced!