The people of „How it should have ended“ have the film „Back to the Future“ Part 2 considered and decided, this film does not need a new ending, but a very different beginning:

In the film, Back to the Future, the first time Doc tests the time machine is by sending his dog Einstein 1 minute into the future. Doc puts his dog in the car, drives it down the parking lot, and zap the car bursts into awesome flaming tire trails. Marty immediately thinks Doc killed his dog, Because both the car and dog disappear. for a Whole minute Einstein was missing until the DeLorean finally comes back from the past. This is why „Back to the Future 2“ sending Marty into the future to see his own future children is impossible. When you travel into the future your timeline is you going into the future. A copy of you is not left behind to live on to 2015, so you can’t go forward to see what your future self would be like. You would jump ahead to a world that lost you in 1985. People would think you had died. But that’s not as fun of a story to tell is it.

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How Back To The Future 2 Should Have Started