I think the best memorial we can see is a two grassyfootprints in the place of the towers. A park environment in the middle of the concrete jungle that shows the world we can afford to let that site sit open for preservation of the hallowed ground that place is.
We can show them that the WTC was not the legit target they claim it to be because we can do the business that was done there elsewhere. We should show the world what moral standing this nation has and make a park out of the entire site with the footprints of the towers only being entered to mow tall lush green grass. I offer that no more respect can be offered to a place where so many died such needless deaths than to consider it hallowed ground only suited for reflection of the events of that sad, sad, day.
That land is far too valuable for that. Besides, to me a failure to rebuild it better and higher is an admission of defeat.
Using your logic, Pearl Harbor should be returned to a quiet pristine lagoon.