For West Side Story (a film I’ve seen possibly 50 times)
My alternate ending would take place after Anita discovers that Maria and Tony had been together (while Bernardo was in the morgue and her parents likely crushed that their son was killed).
Instead of going to Pop’s store, she goes to Maria’s parents and lets them know that Maria was involved with Tony. They take Maria to New York Harbor and put her on the first ship to Puerto Rico, where she enters a convent.
Chino and Anita console each other over her loss of Bernardo and his broken engagement to Maria, and they fall in love.
Pop, instead of telling Tony “there is no Maria”, tells her. “Tony, Maria left you to go back to Puerto Rico.”
Tony says, “But she said she loved me, we sang songs about it and everything.

”
Pop says, “Come on Tony, you’ve only known her since yesterday, and now you’re involved in a murder investigation. Listen, I know someone at the DA’s office, maybe you can get a plea deal.”
Tony goes to the DA, confesses his part in the involvement in Bernardo’s death, and is sentenced for 15 years for Voluntary Manslaughter. He’s let out 5 years early for model behavior. Pop leaves everything to Tony in his will.
With his funds, he purchases a plot of land on Long Island and becomes a duck farmer.
In the mean time, Anita and Chino marry, and she goes to design school, since she was a talented seamstress, and becomes a designer. She opens an upscale bridal shop in New York. Chino goes to college and becomes an accountant. They live in a nice home in Yonkers.
(more like an alternate ending and sequel)