Originally Posted by BiggerJ
Holy crap! I've just thought of the logical conclusion! Biff lives a normal but better life (a better version of his life at the start of the first movie)... and his daughter Tiff starts helping Doc instead of Marty!
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!