We can always let them be stuck during the weeks that they stay in Paris with Ragnar being sick until the fourth season has started, and we know where this is going. As I said on the other post, I think Ragnar might get his door back after his baptism. When he realises that he's not dying / starts using Milliways to not die because even though he feels miserable, Athelstan doesn't want him to die, he will start hatching that plan. It would be just like Athelstan to set the condition that nobody should die...