The Greeks and the Romans had a belief that when an enemy destroys something on principle. You build it again and you build it bigger!
I suggest the towers be rebuilt,
To honor the victims and ensure the world shall never forget. Each tower would serve as a "Sundial" and a permanent shadow (darker cement than normally used) would be built into the ground. The permanent shadow would show the hour in which the plane hit tower #1 and the other building's shadow would reflect when the plane hit tower #2. Both shadows would contain all of the victim's names.