That’s because she is exactly like all the villains who appeared on the show, the only thing that makes her “special” is that she happens to be the final villain to be featured in the final arc (or at least Emma’s final arc).
I don’t think that anyone believes that when Rumple said “final battle” in the pilot, they had the black fairy in mind, like I said she just hapoens to be the final villain. I don’t even think that A&E would have done something with it if fans hadn’t asked them about it.