Santiago Calatrava's improbable residential skyscraper on South Street, a tower of boxes as unsteady as Jenga blocks at a late stage of the game, is dead. And the Port Authority is scaling back the Spanish architect's daring design for the financial district's PATH station. The architectural critics will no doubt bemoan the loss of New York's civic ambition, and wonder why rival world cities such as Shanghai and Dubai have taken up the mantle. (Oh, yes: authoritarian regimes, unbridled capitalism and cheap labor, but let's not go there.) Truth is that the drawing boards are littered with visions of Manhattan that were never realized. Here are my dozen favorites.