I keep seeing talk about another Back to the Future movie. Let me be the one to break it to you, it's not going to happen. There's a very good reason for this, they're all dead.
Okay, not all of them.
We know that the DeLorean was fitted with a flux-capacitor. Also that the flux-capacitor was fueled by weapons grade plutonium.
At the end of BttF 3 we see a train hit the time machine as it runs out of gas on the train tracks. After a few fleeting seconds the flux-capacitor flickers out of juice. Plutonium is usually cased in magnesium with rubber bumpers. In this case, the machine was destroyed and cracked open. There are two possibilities at this point.
1-The machine goes critical after the machine erodes completely and levels all of Hilldale. Depending on the wind there would be a few square miles of fallout. The entire McFly family is incinerated along with Biff and the other beloved characters in the story.
2-The plutonium casing is destroyed and rain runs the poison into the water system. In this case the city survives, however all of the inhabitants are poisoned and more than likely die a much slower more painful death. No telling how long the water table will spread or how long it will take to recover.
Either way Doc's entire family is gone on their train and safe, as far as we know. So there is a possibility of a spin-off of the Brown family.
The real questions to all of this is...did Doctor Brown know?