Wow, there's a bunch of stuff on the Wikipedia entry I never knew about:
:O That original ending sounds pretty cool!
Wikipedia said:The inspiration for the film largely stems from Bob Gale, who discovered his father's high school yearbook and wondered whether he would have been friends with his father as a teenager.
Robert Zemeckis pitched the idea to several companies. Disney turned it down because they thought that a story involving a mother falling in love with her son was too risqué, even if it was a twist of time travel. All other companies said it was not risqué enough, compared to the other teen comedies at the time (see Fast Times at Ridgemont High (1982) and Revenge of the Nerds (1984)).
Sid Sheinberg, the head of Universal Pictures, made many changes to the movie. "Professor Brown" was changed to "Doc Brown" and his chimp Shemp to a dog named Einstein. Marty's mother had previously been Meg, then Eileen, but Sid Sheinberg insisted that she be named Lorraine after his wife Lorraine Gary. Sheinberg also did not like the title, insisting that no one would see a movie with "future" in the title. In a memo to Robert Zemeckis, he said that the title should be changed to "Spaceman From Pluto", tying in with the Marty-as-alien jokes in the film. Steven Spielberg replied in a memo thanking him for the wonderful "joke memo" and told him everyone got a kick out of it. Sid Sheinberg, too proud to admit he was serious, let the title stand.
In the original script, Marty's rock-and-roll caused a riot at the dance that had to be broken up by police. This, combined with Marty accidentally tipping Doc off to the "secret ingredient" that made the time machine work (Coca-Cola) caused history to change. When Marty got back to the 1980s, he found that it was now the 1950s conception of that decade, with air-cars and whatnot, all invented by Doc Brown and running on Coca-Cola. Marty also discovers that rock and roll was never invented (the most popular musical style is now the mambo), and he dedicates himself to starting the delayed cultural revolution. Meanwhile, his dad digs out the newspaper from the day after the dance and sees his son in the picture of the riot.
:O That original ending sounds pretty cool!