I really like the idea of Zelena taking symbols of the classic characters’ traits. So in that context, I agree that Charming is the Cowardly Lion.
I can see how Belle’s brain suggests she might be the Scarecrow, but I think Rumple is the Scarecrow. He was always very smart also and now he has lost his mind (i.e. “brain”) and this could be symbolized by his dagger.
So who is the Tin Man? Regina would be an obvious association with hearts, but since Regina is apparently the main target of the curse, I don’t think she would also be one of the other characters. My next choice was Snow since she certainly has a good heart and the spot of darkness revealed when Regina ripped out her heart could also be a bit of foreshadowing. I initially ruled out Henry because they already dealt with him losing his heart in Neverland and Regina cast that spell to prevent anyone from taking his heart again.
But now I’m wondering if I took the heart idea too literally and Henry might be the Tin Man after all. What if Zelena arranges for Henry to meet a little girl named Dorothy and he develops a crush on her so he “loses his heart” in the metaphorical sense? We saw the storybook surrounded by the smoke at the end of “Going Home” so we expect it to appear again in Storybrooke. The book might be the symbol of his heart. Henry might discard the book for Zelena to take if Dorothy teased him about being too old for fairy tales.
On the other hand, I also like the suggestion that Snow is the Tin Man, Emma is Dorothy, and Henry is Toto.