As a former military planner, I'd say the most important thing for a group planning an attack is to completely understand their enemy's (us in this case) centers of gravity. "Center of Gravity" loosely means a core value or other grand-scale societal aspect that you believe is vital to the ability and will of your enemy to wage war against you. Combined with defining the center of gravity are other principals such as mass, element of surprise, economy of scale, withdrawal from hostilities, etc. While lots of these concepts are not very-well defined outside of formal armies, terrorist groups do go through this thought process in one form or another.

OK, having said this, it's clear that bin Laden (probably the most formal planner of the terrorist big guys at the moment) views our economy as our center of gravity. To disrupt the economy is to attack our center of gravity. On this point, bin Laden generally failed, although his success was that he caused us to divert enormous resources to the war on terror and caused us to make some really bad decisions at a strategic level. He didn't destroy our economic center of gravity, but he definitely influenced it.

I believe another one of our centers of gravity (Yes, you can have more than one.) is our sense of invincibility (It COULDN'T happen here!). Well, it did, and I think any reasonable person would conclude that bin Laden, through a relatively small-scale attack, destroyed this center of gravity. Arguably, we reacted to this defeat of national invincibility by picking a couple of fights, making some really bad decisions, and, to one degree or another, shredding our Constitution. This led to a transitioning from a national invincibility to one of fear.

So, to answer the question posed, focus on what you believe are our centers of gravity. I suspect the bad guys will continue to exploit fear and continue to attack those targets they believe would lead to the biggest disruption of our economy. So, given those parameters, let your minds wander. And, remember, the attacks don't have to be big -- just sensational and aimed at a center of gravity.