It is clear that the game would be ruined if it was an MMO, but imagine a lan system, where you can go through all that but with your friend?
Or even better, a server system like Borderlands. It wouldn't affect the game storyline too much, wouldn't it?
To be honest, i think singleplayer games are slowly getting less and less popular. I have no source to prove this, but it's just what i think.
Therefore, having an epic game you can go through with friends over and over a game just makes it more epic.
Share your thoughts - add ideas. Maybe this can happen, if you agree.