Not only the story, but the complete screenplay from a possible X-Men spin-off movie have been revealed and are now online. Set in the late 80's between X-MEN: APOCALYPSE and DARK PHOENIX, the film was a horror/monster story influenced by John Carpenter's THE THING and would have seen Hank McCoy/Beast facing off with a monstrous mad scientist-turned-"Windigo" in a snow-covered Alaskan village while still battling his inner "beast". Wolverine and Professor X were also to be included, and Mr. Sinister was to be introduced, but it seems that franchise honcho Simon Kinberg refused to even read this completed draft.
For more details and the complete screenplay itself, check out the links below:
Hollywood Reporter Interview
Complete Screenplay