Blueprint for Ground Zero Begins to Take Shape
City and state officials seem to have settled on crucial components of future of site known as ground zero; most officials say publicly that slate for Lower Manhattan is still blank, ready for best and most creative ideas about how to fill gaping hole in financial district's heart; they stress that any final decisions about rebuilding will be made jointly by Lower Manhattan Development Corp, Port Authority of New York and New Jersey, and offices of Gov George E Pataki and Mayor Michael R Bloomberg; still, planners are going ahead with plans for revamping of Lower Manhattan's tranportation system, complete with extensive underground concourse that will house retail stores, and few if any of plans mention either idea of replicating twin towers or of excluding commercial office buildings from site; map; photos (M)