This seems like the most likely approach to me. They are going to expand the definition of a firearm to cover the parts that are harder to print like barrels.
A barrel is just a piece of round stock with a specific diameter and pitch. Gun smithing lathes are called so specifically because they are the right size to make gun parts.
So 3d printed guns will be illegal if they do that?
Not necessarily, but you won't be able to buy the parts to finish a 3d printed frame without a 4473.
This seems like the most likely approach to me. They are going to expand the definition of a firearm to cover the parts that are harder to print like barrels.
FGC-9 already solves this though:
https://www.youtube.com/watch?v=Q1Z9DpoGW7Y
A barrel is just a piece of round stock with a specific diameter and pitch. Gun smithing lathes are called so specifically because they are the right size to make gun parts.