Remember how BB's initial name was "Everything" before Oda went a bit more subtle?
I think there once was a devil holding all the powers that are now DFs inside of it--Everything.
And I think once it was defeated and "robbed" of power, it became "Nothing"--or Void/IMU.
But Imu has been working through the centuries re-absorbing the lost powers. He's been successful with some, the very same he's "lending" out to Gorosei and HK to do his bidding, but he hasn't been able to secure more important powers, like the Yami and the Nika. Maybe these are the key for him returning to full form.
And maybe once he's this close to pulling it off, BB comes in and absorbs IMU instead, re-becoming "Everything".