With the new rules, hardened cockpits, more air marshals, and non-sheeple passengers it would be very difficult to replicate a 9/11 style attack.
It is to our benefit that the terrorists seem to lack imagination as they would be more effective if they diversified their targets. Given the big-government solution to every problem they could bleed us financially by forcing us to secure more and more targets.
Probably decoys....watch for OTHER methods...
Smuggle aboard fountain pens with sarin, epipens with antidote and a terrorist or small group of terrorists would have the whole plane to themselves and no interference as they worked on the door.
The only risk is that a flight crew might, just might, recognized the symptoms with enough time of muscle co-ordination left to radio a mayday.
They may not have a 9/11 style attack in mind. They might simply want to bring down an air liner.