I wouldn't mind mind a shift in focus to Doc and Marty's kids and showing why the real 2015 is different from the 2015 they went into. Marty and Doc would play a role similar to Jeff Bridges in Tron Legacy. MJF and Lloyd will be there but not as the focus. Maybe focus on Jules, Verne and Marlene with a time travelling antagonists. Maybe even Tiffany Tannen.

 In the year 2015, 16 year old Marlene Mcfly fantasizes over the stories of her favorite sci-fi authors such as H.G Wells. She dreams of adventure but is forever stuck in her mundane life. One day her father Martin goes missing when she arrives home. Her father left her a letter that leaves instructions and clues to his whereabouts and car keys. Confused, she is chased by shady men and is rescued by a brazen young man with blonde hair with a sarcastic attitude. He uses an array of high tech gadgets to escape their pursuers. The young man introduces himself as Verne Newton Brown but tells Marlene she can call him Verney.
Verne brings Marlene to the bridge near Eastwood Ravine to meet somebody but Marlene is confused as there is no one there. She is startled with 3 sonic booms and a steam train locomotive appears in front of her very eyes. I was like the stories she has read came to life! Another young man stepped out of the locomotive wearing Victorian era clothing. He politely dressed himself as Jules Eratosthenes Brown and apologizes for his younger brother's rude behavior. The brothers state that their father, Emmet Brown has gone missing as well and the letter left to Marlene holds clues to their whereabouts. Will Jules, Verne are Marlene ever find out what happened to their fathers? What is the conspiracy that  involves their shadowy pursuers? What is mystery behind the founding of Hill Valley?