Disney registered the domains for NationalTreasure3DVD and NationalTreasure4DVD, hinting that Disney wants this to warrant multiple films.
Has been stuck in development due to a script not being complete.
A rumored storyline involved the fountain of youth but that ended up in Pirates of the Caribbean: On Stranger Tides (2011).
Jon Turteltaub said page 47 of the President's book of secrets might not be the focus of the movie.