My brother sent me this link to a slick website that has an architect's proposal for the WTC complex. It's a little on the strange side in some places (sound waves repelling aircraft), but overall is pretty interesting.
I don't think it'll fly (as it were). Last I heard, NY really wants to restore some cross-town streets that were obliterated when the complex was first built, making that part of downtown much harder to get to. Sticking a building 300 feet taller than the originals in the same spot would seem to defeat this purpose.