It would have to be something that "controls" America.
The WTC was our economy. The Pentagon was our military might. The Whitehouse was our form of government.
It would have to be important enough, in their minds, to bring America to it's knees in chaos.
Maybe one plane for the congress, and the other for the White House. That would make 5. Economy (took 2 planes), military (1), law (1), and a "regime change" (1), or no leader to lead the people through the chaos. Maybe?
These guys were camel humpers. They may not have realized a free republic is not the same as a dictatorship, and our information and social structure is not made of a few buildings, a presidential camel, and one dictator.
I wonder if the prescient FBI agent would have gotten anyone's attention if he had told them that the suicidal murderer intended to crash a planeload of passengers and fuel into the J. Edgar Hoover Building in Washington.