1.) I liked part 3!
2.) NO one wants a game about Doc's kids!
3.) NO one wants a game where you can't use the DeLorean!
...
No, of course there are no absolutes, no two BTTF fans are alike, and you may wish for whatever's in your heart. Still, Doc's two little snots do not exactly have a host of followers. That's not to say that these two boys, with 12 more years on their back, a decent re-naming and a portion of individual personality, could not make for good game characters. Still, most fans would just want Doc & Marty back, and that's what's already promised.
As for the time train, I always considered it to be a massively exaggerated asset, specifically designed to destroy the actual spirit of adventure, purposefully ending the movie series forever. After this monstrosity was on the screen, you'd just have to believe that it's over.
So let's just have the DeLorean, which, coincidentally, is ALSO already promised.