One could easily imagine lanes controlled by the end user.

For example, "You pay $60 per month for a 50Mbit cable connection consisting of 1 Gigabyte of low latency traffic and unlimited regular traffic". Then the user can set certain programs to be "low latency" using an app available from the cable company. If they want more low-latency traffic, they can pay extra.

Actual implementation through CGNAT could simply involve having different source port ranges for low latency traffic.