The Shedim will be released, but Sam and Dean will find a way to stop them. However, they don't sleep in the Empty like regular demons, due to their savagery, and begin waking up other demons in the Empty. The Cosmic Entity, wanting to sleep once more, unleashes a horde of demons upon the Earth that would dwarf (hypothetically) opening every Devil's Gate on Earth simultaneously. Amidst all this, Jack goes bad, but is redeemed. Then, I'm guessing, the Cosmic Entity becomes the new Big Bad, and Jack is the only hope they have of stopping it.