9/11 only seemed like a perfect fantasy to people who weren't paying attention. There was plenty of precedent, for the general idea of crashing airplanes into things on purpose to destroy them (kamikazes), the specific idea of hijacking a commercial airliner to crash it into a national landmark (Air France 8969, and in fiction, Tom Clancy's Debt of Honor among others), and the specific idea of killing thousands of people by destroying the World Trade Center (1993 bombing).
We can come up with ways for terrorists to kill thousands of people, and people did come up with such ways before 9/11. I see no way, short of first taking over an entire country, that terrorists can kill millions.
If you think otherwise, then describe it, don't just appeal to ignorance. The "we don't know, so we should assume the worst" argument can be used to support literally anything.