And even though Rumpel took the Beasts place that doesn’t mean that story was non mythical. It is still in the realm of what the show is about. Honestly i think vampires are out simply because of all the vampire stuff there is. Even on Buffy when they had Dracula they made fun of him. Talking wolves have been part of many mythical stories so making Red the wolf does not seem so much of a stretch. She is cursed to turn into a wolf.
Also there are so lines in the sand. They have said stuff like not bring people back from the dead already. All good world building has lines in the sand that is what makes it good.
They certainly existed in LOST. The only abilities people had were things like in peoples minds, talking to the dead and whatever Walt was doing, astroprojecting or whatever that was. No one ever through fire from their hands or anything. I don’t think that would have worked for that world where the biggest mystery was the island.
PS. When did they say they would bring comic book characters?