Serena Williams is pregnant, and will continue to be pregnant through the next two Grand Slams (just in time for Maria Sharapova's return). And I doubt she'd want to play the 2017 U.S. Open, too. Even though Serena was reaching the point where she couldn't be favored to win every Grand Slam tournament she was in, she demonstrated last year that she was still formidable and victorious. Now that she'll be on maternity leave, clearly the door is open for some other women to win Slams.
Serena is currently ranked #2, so I think anyone in the top 15 would have a shot (especially with sister Venus Williams sitting at 12).
Wozniacki and Halep -- now's your chance. Seize the racket - and seize the day.