If that is how you feel then why not just rewatch the movies? I understand what your saying and I'm sure there's some people who just want it to be Marty and Doc but TBH I think this game is a PERFECT chance for the series to get some new life and bring some more story to the true fans.
I'm not saying leave Marty out of the story completely, maybe just have him as a side character who gives quests? I just think this is too good a opportunity to add a few more layers into the BTTF story instead of just having the same old stuff we've all already seen.
Maybe if the story unfolded that Doc spliced some of Martys DNA into the kids (I know it sounds weird but we can leave the specifics to the story writers), that way the kids are pretty much new Martys so the old school fans are happy, and all the players like me are happy at seeing some fresh air into the series?