I think that being placed on a terror watch list should be the same process as an arrest warrant. A LE agent or officer presents facts to a judge in an affidavit. If the judge sees fit...it's approved. Make it appealable.
Allowing the EXECUTIVE BRANCH (law enforcement) to make lists of people they deem dangerous is one thing...allowing that list to be denied rights based solely on being placed on that list is another.
There must be checks and balances.
Executive branch (LE) should present facts on each person to a judge (appointed by legislative branch) to have the person listed.
Otherwise....it's allowing the government (police) to say "You can't buy a gun because we say so". Slippery slope there.