Yes, I read it about 15 years ago. What surprised me was the vast difference between the movie version and the book. I think Margaret Mitchell really didn't like Scarlett.
No, if Mitchell had written a sequel the movie hype would have raised expectations so high that her book couldn't have been a success. She had a somewhat "rosy" view of antebellum South, and by the time her book would have been published, opinions and views were changing. She depicted Rhett and Scarlett as survivors, albeit not very attractive ones. I don't know where she could have gone from there, and no reader would have believed either character as repentant and kind.
I like the fact that Scarlett's overriding concern was getting home to Tara. She declared, "I want my mother. My mother needs me."