I think Dead Space 3 got the co op formula right. Totally optional, and adds to the story, but isn't necessary. I don't like co op games very much (where the partner becomes AI playing alone), but I do like the option of playing co op if a friend comes over. Even if it messed with the story routes a bit, it would make it more interesting. I liked RE1 and 2's multiple story lines, endings, options, unlocks, etc for different characters taking different routes. And I liked being able to "affect" the other game through my choices. Even the zapping system from RE0 would make an interesting alternative to co op. Running around with an invincible ally who gets stuck in weird loops doesn't appeal to me, but I think there's a few ways that they could please the co op fans while still making it feel more "classic".