I personally expect the next attack will involve radioactive material in some way. I'm not ruling out actual nuclear devices, but I think a conventional explosive dispersing radioactive material is more likely.

It'll be done in a way that hasn't been addressed much, or a way that simply cannot be identified in advance. An ordinary vehicle at a sporting event or similar large gathering, or a small aircraft flying over a population center, apparently making a normal approach to a small airport until it diverts at the last minute.

And I'm sure that if I can think of it, so can they.