I think this is a loaded question, that assumes we can solve all problems by passing more laws.
Since you mention Orlando, let's ask this question:
Would a federal terror watch list law have prevented this guy from legally buying the firearms he bought? No. He was not on the 'terror watch list'.
The problem with the watchlist idea is how to do it judiciously, and have safeguards to remove someone on it by mistake.
We have federal laws that require background checks to legally buy a firearm. If you are a felon or convicted of a domestic violence misdemeanor, if you are under a restraining order, or have an above conviction pending, you can not legally buy a firearm from a registered dealer.
Any criminal who wants to buy a firearm today can do so illegally.
Government needs to keep bad guys who commit crimes with a firearm in jail longer.