In an interview with Collider, National Treasure franchise producer Jerry Bruckheimer talked about the progress being made on a third installment in the Nicholas Cage starring series:

“Those are the hardest movies to make because they’re all based on clues.  It’s so difficult to work them out.  But, we got a first act in [on Friday], and Jon [Turteltaub] was very excited about it and I’m very excited about it.  Hopefully, the first act will lead into the second act and the third act, and we’ll get this thing going.”
National Treasure was a successful franchise for the Walt Disney Studios during the Dick Cook era. The last installment in the franchise was released in 2007. While Jerry Bruckheimer's first-look deal with Disney was not renewed, he continues to work on films from existing franchises such as National Treasure and Pirates of the Caribbea.