I think they've been setting up the Spirit King and Squad Zero for ages now. That seemed to me to be the obvious next phase of the story. Aizen's whole purpose seemed to be to become strong enough to take on the Spirit King. And if he didn't do it, Ichigo obviously has. It just becomes a matter of introducing whatever the conflict between them is.
My money is on the Spirit King kidnapping one of Ichigo's female friends. And Ichigo must break into the Spirit King's realm to get her back! But first, Nel has to train to learn new powers!
I really didn't think going into the Heuco Mundo arc that Ichigo and Aizen were going to tear down in this arc. I thought Aizen was going to sacrifice the Espada for whatever his actual purpose in this arc was and that he and Gin and Tousen would succeed and leave to be fought in some future arc.