i think a way to resolve the ¨unseen content ¨ problem is to have an after-game 'section' that can only be completed/experienced if you have seen enough of the game.
it could be a book that fills up with lore/fluff, a HARDCORE playable section that can only be beaten if you really know what you are doing, or simply a box of clues that help you figure out what really was goin on in the story.
this would also require a system that, upon re-plays, helps you avoid paths that had already been traveled or partially travelled.