I don't think that the same scenario would happen again, given the new technology aboard american airlines after the attacks. For example, the door to the cockpit of any plane is bolted shut before and after anyone gets on and off the plane. The pilots are specifically instructed that no matter what that door cannot be unlocked. After an attack like 9/11 it is easy for them to know the destructive damage that can be caused by that single plane.
However, I think it is (unfortunately) very likely for another large scale attack to happen sometime soon. I don't think that it necessarily has to be in the form of civilian casualties, but I think it is very likely to be a cyber attack. As more and more resources are poured into this by ISIS (as we saw them hack several US official social media profiles), it wouldn't surprise me if something were at least attempted in the form of a cyber attack. Especially because the threat isn't truly recognized until after it is too late and therefore there is really not enough cyber security where there should be, because no one has really exploited it too terribly. The bad part of this is that its always easier to fix the security problems after they are exploited.