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Ragnar actor Fimmel has previously said he felt ready to leave the series after season four, and it had been an easy decision.
He had told Collider at the time: "I’m not one of those actors that live and breathe the character, and take it home. It was such a great experience."
The show's creator, Michael Hirst, had known all along Ragnar was going to die and his sons were going to take over his legacy.
Similarly, Lagertha's role in the series was coming to an end at the time of her death.
While there is no reason why their early story could be told in a prequel, there is no guarantee the actors would want to reprise their roles.