The approximately 3-acre, 1.2 million-square-foot site interests developers because an ordinance, introduced in March by a downtown alderman, would allow the site to be zoned to accommodate one of the tallest skyscrapers in the city, according to city officials. If the measure is approved by City Council, the building’s square footage could be replaced by a tower of more than 2 million square feet — enough space to reach 100 stories or more.