Well, you said, all they have to do is....
Why would any State have any problem with defining NBC though a Constitutional Amendment?
Depends on what the meaning of "NBC" is.
My personal view is that nothing will ever happen, one way or the other, regarding clarifying the meaning.
I don't know how much clearer it needs to be...jus soli AND jus sanguinis.
The founders didn't want any freaking foreigners as President.
The biggest hurdle will be getting Congress to pass an Amendment. Unless Congress passes a real doozie, why would any State not sign on?
The best opportunity is between POTUS elections and neither side having an ox in the ditch.