Everything but the roof
peter bodo. tennis.com

The USTA unrolled a massive, $500 million, seven-year renovation and expansion plan for its Billie Jean King National Tennis Center. What, $500 million and no roof on Arthur Ashe Stadium?
"We simply do not have a roof design that works at this point," USTA CEO Gordon Smith admitted. "We have been working on that, and will continue to work on it with engineers and architects."
Building a new, roofed stadium comparable to Ashe would cost, according to some estimates, close to a billion dollars. Retro-fitting a roof to Ashe, aside from the engineering problems it presents, would fall more within the $125 to $250 million dollar range.
According to Smith, an investment of that size in a roof that may not even be used during a semi or a final for years would leave the organization without the resources to complete many of the improvements unveiled today—see the image at the top of this post. Those include replacing Louis Armstrong Stadium and the connected Grandstand (a favorite of many in-the-know fans) with a brand new Louie in the northeast corner of the grounds, and a new Grandstand on land that is currently part of the USTA's lease but unused. (Unlike Ashe, the Armstrong/Grandstand complex is nearly 50 years old and at the end of its useful life.) The USTA also resisted putting a roof on the new Louis Armstrong Stadium because it would have to hold at least the same capacity as Ashe to make it feasible to send patrons from one stadium to the other if rain came.
This, and some of the other changes, will also have a comfort dividend, as improving traffic flow and relieving congested areas has been a target of the improved site since the get-go. Other enhancements include widening some of the walkways (which involves moving some of the southern-most courts), a few new buildings, and at least one very welcome and innovative feature: A covered, elevated walkway and seating area extending out from Ashe, from which fans will be able to watch players practice.
Those of you who have attended tennis events in person know how staking out the practice courts has become a wildly popular fan pastime, even though the practice courts are usually located in the least crowd- and traffic-friendly areas of a venue. "Fans will be able to sit and watch their favorite players practice," Smith said, "and we're even going to post the practice schedules."
Some of these arguments will not placate die-hard roof fans (or USTA critics). I've said that it's imperative in this day and age for the tournament to finish on time, there's just no excuse to run over because of rain. But I also think the USTA got blind-sided by this sudden rush to roof, and think the outfit is choosing the wiser if less popular road. Tennis is still, after all, an outdoor sport, or one best enjoyed as a spectacle outside.
It's also heartening to know that the new Louis Amrstrong Stadium will be built to accommodate a retractable roof (the way Court No. 1 at Wimbledon was), for the day when Ashe has outlived its usefulness. Until then, all we can do is pray for sunshine.