I'm sure they could work on something but I personally feel it would be much more interesting for Marty (& Doc to a extent) to take a backseat and flesh out some new characters. The fact is we know very little about Docs kids, so they are a blank canvas for TT to work there magic and create some very likeable charactesrs.
With Doc and Marty, I mean how much more do you want to know? Who tought Marty how to skate? What color Doc's hair was in the past? I know what your saying, but when you think about it I think it would just be annoying if the game was just finding out every little detail of there lives (though I'll admit I'd love to see how they meet).
You also have to remember that TT are making this to hopefully make money (which I hope they do since it means higher chances of more BTF games). The train has already been proven that it can be successful in video games, and I think there's more a chance of sequels etc if Doc's kids took the stage at the very beginning (the "Video Game" perspective)
Anyway I understand that everyone opinion is different but personally I feel that the best of both worlds (the fans are happy and TT can make profit) should try to incorporate some of these ideas.