So, here is how I think they could reboot the Back to the Future franchise.
I was thinking of just having a contemporary kid off the streets find the Delorian in storage somewhere, and he breaks into it and car jacks it. Then the cops come after him and in a high speed car chase he accidentally hits 88 mph and is whisked into the far future.
In the future he meets a girl who shows him how to manage thing in the future, they have fun, and eventually they fall in love, but she ends up sending him back (knowing something he doesn't).
When he arrives back in his present, and evil character (from the future) recognizes him and tries to kill him. But the kid survives and then goes back to the future to figure out what the hell is going on. That's when he finds out the girl has been mysteriously transported to the past by an evil scientist. So he goes into the past and finds an evil Doc Brown, a true mad scientist, has been wreaking havoc on all the time lines.
Turns out the boy is the descendant of Marty McFly and the evil Doc Brown is trying to erase all traces of Marty and his bloodline from every possible timeline (for obscure reasons).
The girl and the boy decide to stop him together and find the point where Doc Brown became evil and change events so that they can reset all the time lines where Marty's progeny isn't being hunted down.
(That's my idea for a fourth film, anyway.)