When it comes to CS debate, I stick to two places: here and my tumblr.
My opinion on Milah, I think what we need to know about her story is done. To be honest, I don’t think we were meant to know more about her character in general other than her main two character identifiers: Rumple’s wife and Hook’s lover. Bae’s mother also but we knew very little about her background in general. This makes me conclude that she is like the Blue Fairy = she is a plot device character. The first episode we met her, it was to explain the rivalry between Hook and Rumple. Second episode she appeared, to explain why she despised Rumple’s so much.
If we learned anything else new about her, it was from Rumple or Hook (the drawing thing). I do remember writing a CS/Milah fanfic trying to fill in the gaps of Hook/Milah relationship but I don’t think we need to see anymore about her character, especially since Hook put her into the “first love” category. Milah = Hook’s past and Emma = Hook’s future.
I’m okay with that. Now where I think Milah might come up again in next season, will probably to parallel the Rumbelle relationship since they are currently married. I want to see how Belle deals with Rumple’s big lie and if she behaves like Milah in how she treats Rumple.