Throughout all the films, TV series and both movies, the tannens are pretty much always villains. The lone exception would be the version of the present we see at the end of parts I, and II and beginning or part II and episode 1 which we see very little of but doesn't seem to be a protagonist or antagonist.
It seems in this next episode, Doc is the villain and the Biff we see will be the most wussified version. Since Doc will be the main villain (back to the future is the only series that could have a main character be a hero so long and then turn him into a villain!) would you like to see Biff as a secondary villain (maybe in the one clip we see of him, he's putting on an act) or as a protagonist actually helping a mcfly for the first time for a reason other than money.