Characters not getting what they wanted and what happened after the movie ended
I've seen the movie a few times and read the book but after my most recent watch I noticed the running theme of characters losing by not getting what they wanted.
Scarlett - wants Ashley, then Rhett
Rhett - wants Scarlett's love and to raise Bonnie
Melanie - wants another baby
Ashley - wants Scarlett a little? success? growing old with Melanie?
Bonnie - wants her mother's love
Charles - wants Scarlett's love
Belle - wants Rhett
Suellen - wants Frank
Frank - wants Suellen, then Scarlett's love
Everyone - wants to win the war
Mammy is the only one who doesn't seem to want anything, though perhaps at a stretch you could say she wants Scarlett to behave. You can also add that Aunt Pittypat wanted to know what Belle's place is like, Prissy wanting to be respected as someone knowledgeable about birth, and the couple who wanted to buy Tara, though these were only in one scene each.
I'm sure it's no coincidence that on the same day, at the the BBQ, that Rhett states that the war will not be won and that Scarlett is confronted with the fact that Ashley loves and is going to marry Melanie. Yet they still enter the war and Scarlett still pursues Ashley. Double foreshadowing.
From this theme, we can see that Scarlett and Rhett can never be happy together. They can't both want each other at the same time. As soon as Scarlett realises it's Rhett she wants at the end of the movie, he no longer wants to try with her, even if some feelings might remain. This means after movie ends, Rhett divorces Scarlett. Rhett truly gives Scarlett everything she ever wants, including the freedom to pursue Ashley once he became a widower as this is what Rhett thought she still wanted.
This is why Margaret Mitchell didn't write a sequel: Rhett and Scarlett can't have a happy ending. The sequel would have had to be Scarlett chasing Rhett, only for them not to end up together, unless a new theme was chosen. However, this then would have diminished the first book and wouldn't have reflected the civil war being lost and staying that way. Rhett joins the war when he knows it will be lost and similarly pursues Scarlett when he knows he she does not want him. The end of the story where he gives up on Scarlett and accepts the situation parallels everyone accepting that the war has been lost and moving on.
As for what happens next, following the theme, Scarlett would then marry Ashley, only to fully realise how weak and boring he is compared to Rhett. Ashley did want Scarlett a bit (or didn't bluntly tell her no anyway) but by the end of the movie he only cared about Melanie, which would continue throughout his marriage to Scarlett. Finally Scarlett would have to accept that she had lost both Rhett and Ashley. Rhett ends up spending more time with Belle, but doesn't marry her (which Belle wants). He never gets over his feelings for Scarlett but does not want to be with her either as he doesn't find her new feelings for him genuine (similar to how she was pretending to want him in the jail scene). Things have just changed too much for the relationship to work. It's likely he would marry someone else and his wife would never get his full affections.
If you consider that the first half of the sequel miniseries "Scarlett" has Scarlett and Rhett divorce, Scarlett not wanting Ashley when she is finally able to have him (though they do not marry), Scarlett pursuing Rhett (only for him to leave her), Rhett marrying someone else but being hung up on Scarlett, and Scarlett unable to have Tara (what she wanted at the end of the movie), then I actually think they did a pretty good job continuing the story. Things fall apart however when Scarlett moves to Ireland and ends up winning in everything and the end just goes completely off the rails. Unfortunately Alexandra Ripley and whoever made the miniseries had no choice but to give everyone the happy ending they thought they wanted but I like to think they would have gone their separate ways again soon after. I'm yet to read the book of the sequel but it will be interesting to see if they follow the theme better than the miniseries.
(Edited to expand some parts)