The popular cultural notion that airliners could be used to crash into the World Trade Center had been floating around for decades prior to The Lone Gunman. I had first come across it in 1989 or so, but I suspect it goes all the way back to 1973. It wasn't a unique or new idea.
Assassinating the president is as old as Rome, but if there was a TV show from March, 1963 where the military-industrial complex, mad at the president for not escalating Vietnam, decided to kill him by hiding a gunman in a building along his route in Dallas, We’d still be talking about it now.