I’ve done a bit of research into this topic, to see exactly how we could be affected by any change in ATF’s stance toward type 03 Federal Firearms Licensees.
Stepped up inspections of C&R FFL holders would be the most obvious way to make licensing uncomfortable enough that a lot fewer people would want to do it.
They could also change the C&R terms. Title 18, Chapter 44 pretty much leaves this up to the Attorney General, but the standards for a firearm to qualify as a Curio and Relic are part of the Code of Federal Regulations, and to change it would require going through the rulemaking process. This can be done without an Act of Congress, however, but ATF can’t (at least not without getting into areas of questionable legality, so that’s not to say it couldn’t happen) unilaterally alter the requirements for being on the list.
Eliminating the Curio and Relic license, changing the fee, or adding requirements to own a safe or other such things, would require an Act of Congress.
So my guess is, if ATF is going to start publicaly going after C&R licensees, it’ll likely involve the rubber glove treatment using the inspection powers they already have. Make you all your C&R holders out there are keeping your paperwork in good order, which you should really be doing anyway.