When Amy returns to Nick after killing Desi and accusing him for her rape, the police and media supports her and Nick is left with no option but to accept her. Later Amy becomes pregnant and Nick is happy about it and starts loving Amy all over again.
Why does it need an alternate ending?
Now let's accept it, Gone Girl was one of the best movies with one of the worst endings. Amy Dunne was a complete maniac and nightmare and still she manages to get away after all she did. And even after ALL this at the end Nick decides to live with her! Which is the most absurd thing anyone can ever think of. So don't you think it would have been better if something happens, which somehow, makes up for her actions?
When Amy is robbed and has no other option, she calls Desi Collings, her ex lover, who agrees to hide her in his lake house which has surveillance cameras. Amy decides to stay there for a while, but when she watches Nick on the interview he gave, begging her to come back and telling her that he loves her, Amy knows that the Nick she married is back and decides to return to him. For doing this she plans to sleep with Desi, accuse him for her rape, and then kill him. She succeeds in doing so and returns to Nick. The police and media supports Amy but Nick and Officer Boney don't believe her and they start searching for evidence against her. When Officer Boney checks the camera footage of the house, she sees Amy entering happily and staying in the house and doing everything she wants to do with Desi only coming in at night with food for her, with no sign of injuries on Amy the next day, or even in the morning of the day she came back. Amy ignores all this because when she first came to Desi she was just looking for a refugee and was sure that she will be killing herself at the end and therefore didn't look out for the cameras, making all these mistakes. When Boney finds this she decides to use it as evidence against Amy and then further goes to talk to all the people who suffered the same because of her, including her ex boyfriend, Tommy O'Hara, whom Amy accused for her rape. Boney takes everyone's statement and based on this and the camera footages, Amy gets convinced. When this happens Amy gets a massive shock and all of a sudden she wakes up in a mental asylum.
It was all the work of once highly imaginative...Amazing Amy.