So it is confirmed the other witches will be known characters, so let's guess who they are!
Here is what we know for sure:
To cast the curse, the witches are Gothel, Drizella, Regina, and 5 other unknown witches.
In present day, the witches are Gothel, Anastasia, and 6 other unknown witches (5 of them are assumably the same as the ones to cast the curse; Drizella and Regina will not be in this coven, as far as we can tell).
One of the witches will be related to The Haunted Mansion.
Here's a quote from an article, proving they will be known characters:
Will we be able to recognize any of these new witches? Kitsis: We'll definitely recognize a few of their names, absolutely. These are witches from iconic fairytales or Disney lore.