Modern Architecture tends to embrace sustainable energy and efficiency as the cost of 'running' a modern massive structure escalates.
When pre-occupying oneself with concerns of history, heritage and monument it's important to remember/be aware that Architecture is about social design, growth and direction, not about making 'stuff'.
Society's Architects are charged with the concrete [pun] fabrication of the Community's future direction.
It's irrelevant that a proposed replacement to the WTC be a bigger or lesser 'target' to potential attack, but that the Society it 'monuments' not be swayed from the legitimacy of being that WAS the WTC.
When I studied Architecture I was initally bemused that 3 of the included subjects of the 44 were Sociology, Psychology and Philosophy...but 'doing them' I soon understood their importance was greater than 'Materials and Structures 1', etc.
We all create our environment at one level or another.....whether it's by grafitti, littering, murder, rape, having kids, building shopping malls, helping in soup kitchens or bossing the corporate machine.
The only really appropriate 'thing' for the WTC site is simply another WTC structure. Its very existence is the tribute/monument, and all that'd be 'good' to be added would be a commemorative 'stone' listing the names of those who died.....nothing OTT or dominant, just simple, genuine, and honest.
Future generations do not need the event force-fed, just not forgotten.
Future generations is what structures of this sort are all about [once one ignores the corporate investment profiteering, that is]....not so much the 'past' but 'future heritage'....