Perhaps the investigation into his own record by the Clinton library staff is responsible, perhaps Hillary knows he's a liability, or perhaps it's just that the White House isn't big enough to hold two such massive, power hungry people at the same time, but US News & World Report says that if Hillary is elected, Bill will not be the First Lady:

"We hear from key Clintonistas that there's an alternative to having Bubba around all the time, stealing the limelight. It's having Chelsea Clinton, the all-grown-up daughter, take over the East Wing. The scenario: Chelsea would move to the White House to handle social functions because the former president would be too busy with his surrogate work for his wife and his ongoing efforts for his foundation, which he'd probably move to Washington. And to keep out of Hillary's hair, he'd step up his already busy travel schedule out of Washington and the country."