There are quite a few characters that I would like to bring back. Unfortunately most of them are dead. Andy, Anna, Mary...
I think Agent Henrikson would have made a great hunter. Perhaps he had somehow survived Lilitth's attack on the police station and then he went underground and became a hunter.
I really like Madison and the actress who played her, Emmanuelle Vaugier, so I would like to her back on the show perhaps in an alternate reality kind of situation where she and Sam are still together.
I think the one I want to come cack the most has to be John Winchester. Perhaps in a flashback episode. I just reawlly hope that we get to see Papa Winchester again before the show ends.