If I were the folks who did the attacks of September 11, the next round would be smaller planes, equipped with whatever explosives I could sneak in or make.
A single-engine Cessna loaded with an ANFO/ball bearings mix could do a lot of damage. Nothing on the scale of what those airliners did, but plenty do do a lot of harm..
If there is to be another attack on America, it will be aimed at our war-fighting capacity. Attacking a civilian target will only stiffen our resolve and piss us off even more, and there is no need to do that. If they want to hit us here, they will attack a naval or air force base with an old Soviet suitcase nuke.
But it's more likely that they will attack a European capital to discourage that country from backing our effort. Look for a weapon of mass destruction to be detonated in London, Paris or Berlin tomorrow.