Originally Posted by mathman77
And Doc, living a friendless life, builds a time machine and goes to 1931 to make sure his younger self has a friend in the future, causing the McFlys to carry on into the future. Huge paradox, but three Docs in the same place!
But that would result in Post-Movies Doc starting to fade away like Marty, since the fading of Marty has set Doc's past self from the hiring on an assistant onward on a new path. Fading Marty and Fading Doc could still set things right, which would cause Friendless Doc to fade away himself.
That could work, but it's probably better to keep things simple, with a brief sequence showing Better Life Biff and Doc's Assistant Tiff in an alternate world that isn't too bad but pisses off Marty ("Not on my goddamn watch!").