6. Cthulhu
Who They Are: Created by legendary horror/science fiction writer H.P. Lovecraft, the Cthulhu mythos are some of the most iconic and beloved in all of literature. The mythology is as rich and dense as you could possibly want, teeming with a complex set of characters and ideas. Having Emma and the gang interact with the Great Old Ones, as well as taking them to Cthulhu's home, the sunken city of R'lyeh, would be a magnificent treat. The Once producers seemed to have taken to splitting up each season into two separate arcs, each played out over the first and second half of each season. You could absolutely focus a strong 3-4 episode arc on the Cthulhu mythos and weave the story out of the major arc being played with, opening up the possibilities of more literature characters, and even more of Lovecraft's mythology, for future seasons.
Who Should Play Them: If there's any man of pure talent that can give voice to Cthulhu himself, it's Liam Neeson. His commanding, distinct voice would be incredibly appropriate for Cthulhu, as well as to give him some real depth and fidelity. If Cthulhu ever actually did make his way to Once, it's likely that he would have a very direct connection to the world and characters, and would require someone with the gravitas to pull it off. Neeson has such an incredible, naturalistic voice, but he can turn on the fury at any moment. He can wistfully bounce between the restrained, yet dignified sound of Aslan to a much more intense, driven characters he's played in films like Taken. This is the kind of actor you need to give Cthulhu life on Once, and there's no better time than season four to do it.