That is all fine in theory but realistically speaking I just don't see Carrie Fisher being capable of delivering this great villain you are talking about.
I think they would be just wasting the whole trilogy potential if they pursued the character potential you are talking about.
And this is supposed to be 30 years after the OT. How could you possibly expect that some torture or some near loss of Han that happened 30 years ago would lead Leia to the dark side.