Maybe Big finally realised that he really,actually just wanted great no-strings sex (with none of Carrie's dramas). He and Sam end up having a massive drunken rendezvous and when Carrie finds out, Sam sees it as a great opportunity to finally tell her what she thinks of her years of wingeing about him/Berger/Aiden/any other man she's ever met, and runs off into the sunset with Big for a lifetime of mutual great no-strings sex. No 'I love you's', just a happy life together, living independently but under the same roof/sweaty sheets.
Harry and Charlotte have a massive secret that both are too embarrassed to speak to the girls about. Harry's law firm has been sued by a wronged client and all the lawyers have been let go, they're facing bankruptcy. Harry helps Carrie to get a fair deal in the divorce, and she is finally able to repay Charlotte for selling her original engagement ring to help her buy her apartment. She is able to help Harry set up again by himself and becomes a sleeping partner in his firm - finally a good investment that doesn't involve shoes.
As for Miranda, she and Steve had another baby quite late so are still in the thick of parenting, whilst juggling her job in her own firm and him running a chain of bars on Manhattan.
I've been planning this series in my head for a while....