How about this?
It's 2015 as it is now, and Marty knows this is not the future he visited - no hovercars, no food-o-matic machines, and no home energy reactors. Marty needs to fix things to how they should be, but the Doc's off time traveling, meaning he needs to create some kind of signal for the Doc to notice. He goes to the Doc's old house and finds a prototype flux capacitor. He struggles to get it to work, and finally, desperately throws it in to the transformers at a powerstation, causing an explosion and injuring himself. The Doc in the future gets the message and travels back to stop Marty getting injured. Marty explains the situation, and they travel back to the 90s, where post-trimetravel Marty (the original 1985 Marty) is having a melancholic reaction to his travels and feels that nothing is worthwhile. His recent marriage to Jennifer is failing, and he seems oblivious to the fact she is pregnant even though it's obvious to the audience and even Doc. This depression, through a myriad of small encounters, altered the course of time.
So future Marty and Doc take past Marty on a journey through time, and on a journey of self discovery (like A Christmas Carol crossed with Bill and Ted), inspiring him to fix his recent yet crumbling marriage to Jennifer, and to build the future he saw. 90s Marty will need to fix his relationship with Biff (now almost 60, and thinking of retiring on his small Auto-detailing business), and reveal everything about his time traveling past. In this reality, Biff had mellowed out and matured, so agrees to go along, and Marty promises him that if he invests in things like this new hover technology, it will secure a better future for little Griff (incidentally, Griff grows up to be a nicer person because of his now not-grumpy and more friendly grandfather).
Returning to 2015, everything is as it should be, with a slight twist (like it wasn't Jaws 19 that was the blockbuster movie). As the final line before the credits, Marty quips something like "I'm back", or "It's good to be home", but a more relevant to the events of the film.
21 Oct 2015, 3:53 am
