I think this is my favorite idea so far: http://users.jagunet.com/~disrael/weekly010919.htm
There are all sorts of grandiose idea going around - in more or less two camps. There’s the “build 'em back to spite the bastards” crowd, and the “build a park to honor the dead” crowd.
My favorite (and what I feel is most likely) would be like the 6th floor museum in Dallas - a floor or two of whatever building ends up on that location given over to a multimedia presentation on the history of the attack.
I just hope the presentation won’t include footage from an interminable, futile war that the attack triggered.