Although there will be a final bonus scene in the dvd of series 5, lets examine whats most likely happening after the shows done, assuming the devil is indeed dead and they arent dreaming:
Hal, Tom and Alex are probably trying to live supernatural free lives. I would like to think Hal and Tom go back to work at the Hotel, maybe Alex joins. I would like to think Alex goes back to her family, maybe Tom reaches out to Alison, maybe Hal reaches out to Lady Mary, Adam, others. I would basically like to think they are having happy normal lives. HOWEVER if only those 3 turned human after the devils defeat and not all vampires, werewolves and ghosts, they might have that to notify. I doubt Alex would want to become a ghost again, she didnt like it one bit. But if the vampires and werewolves still exist, I would like to think perhaps Hal wants to become a vampire again, to regain that god like status of his, to become immortal and never age and never die. And I would like to think Tom may want to become a werewolf again, as he goes back to saying how the curse was a blessing, a gift. Also there is the question of if the creatures in Purgatory were affected by the devils demise. All the souls(annie, mitchell, george, etc) and the men with sticks and ropes were affected or not. If not, then there could still be issues. If Hal, Tom and Alex only became human and everyone else still has their supernatural curse, there will always be these issues. Those who followed Kemps teachings of wanting a world without the supernatural curses might want him out of purgatory to continue his mission, those vampires who worshipped Mr Snow and The Old Ones might want them back to life as well as Herikk if there is a chance they could be brought back from a pile of ash to rampage and continue the true nature of the vampire and feed on all humans. And there is the big one, the devil himself, the men with sticks and rope, and any other follower of satan might want him brought back to resume the apocalypse, so it would probably be best to destroy rooks body since the devils essence still resides within rook meaning he could come back under the proper conditions. So unless every single supernatural including those in purgatory became human, then there could still be some supernatural issues, but I would like to think Hal, Tom and Alex take care of some loose ends before they finally can live in peace.