Closing both Heaven and Hell, bold thing, I can definitely see this being "end-game" for the series. It would then give the humans a chance to really direct their own lives.
However, I do wonder what would happen when someone dies, no heaven or hell to go to, do they go to purgatory? Or does no one ever die at all really? So many consequences to closing both heaven and hell. But it would certainly give Heaven a chance to sort out its mess. Very interesting story line threads here, I can see this going on a couple more seasons at least.