I consider coordinated vehicle bombings at heavly populated targets (malls, disney land, etc) to fit the spectacular category, especially if 6 of more. Smaller Bali-type operations (shall we add the recent OK 'incident') would be showy and hard to defend against, but would require more than a dozen to be spectacular IMHO. Less than that would show weakness by the terrorists in that they cannot hit the US with strength, like 911. This has been their tactic of choice, with 911 being an outlier to their MO.
Had to look it up:
**** with 911 being an outlier to their MO. ****
outlier, n
1. One whose domicile lies at an appreciable distance from his or her place of business.
2. A value far from most others in a set of data: “Outliers make statistical analyses difficult” (Harvey Motulsky).
3. Geology. A portion of stratified rock separated from a main formation by erosion.
Agree with your thoughts for the most part... Not completely convinced we can call 9/11 an outlier, as we have no other successful attacks on America to compare it to. Clearly 93 had the same massive motives. The Millennium attacks were also poised to be very large in scale. America simply does not have the pool of eligible Muslim extremists (right now) that most of the other places we have seen attacks do. If you can pull off small attacks every 6 months to a year, why not go with 4 backpack bombs that kill as many as one well placed truck bomb?
I don't think they are in a similar position regarding the U.S., unless there are really thousands of sleepers waiting for activation... a claim I wonder about at this point. If the upper management can only gather resources to pull off an attack/s once every few years here, it seems they would have much greater interest in 9/11 style, rather than London style, planning.