I wonder how many different mythologies they'll borrow from? So far the only thing that doesn't seem to be an actual fairytale is the thing with King Midas.
If the room of portal doors that Jefferson had to go through are any indication, (taken from Once Upon A Time Wikia) some of the potential worlds to involve stories from may include (but are not limited to):
Grey Door: ?
Pink Brick Door: ?
Green Door with a Hole/Mirror: ?
Brown Door with Trees: ?
Blue Door with four Holes/Knobs (with possible scales): ?
Medieval Brown and Grey Door: ?
Oriental Style Red Door with Golden Knobs: ?
Green Curtain Door with the Oz Logo: ?
Mirror Door (known as "Looking Glass"): Wonderland
Coffin-like Door (or Corridor): ?
Normal Door with a Keyhole: ?
Until now, there has been revealed only where one of these door leads, but there are many theories related to the other doors and worlds, such as:
Land of Oz (Green Curtain Door with the Oz Logo)
Neverland (Normal Door with the Keyhole or Green Door with a Hole/Mirror)
Imperial China, where Mulan would appear (Oriental Style Red Door with Golden Knobs)
Atlantida (Blue Door with four Holes/Knobs)
Camelot (Grey Door or Medieval Brown and Grey Door)
Fairy-Tale Land (Pink Brick Door or Brown Door with Trees)
Real World (Grey Door or Normal Door with a Keyhole)
Some kind of Dark/Evil World (Coffin-like Door)