Johnson calls the project "the most complicated stadium ever attempted." The concept turned out to be so ambitious that some features have been cut to meet the budget. A natural ventilation system, with operable window walls, is gone. Fixed-roof cantilevers have been scaled back and the skin simplified. But the stadium's sacred cows—the daring convertible roof, the halo scoreboard and views to the city—remain.

Erleen Hatfield, a principal of Buro Happold, the stadium's structural engineer, knows Johnson can deal with the changes. "Bill is an expansive thinker, highly creative and super pragmatic," she says. "If one of his ideas doesn't pan out, he has 10 more in his pocket."

Johnson is still looking ahead, saying, "Next time, I want to focus on opening the concourse, not the roof. You always need to question the status quo."