I always hesitate to say they need to move to a new engine because that could seriously mess up the pristine gameplay they have nearly perfected. They would need to completely rewrite the entire system. Not saying it'd be impossible but unless its something they've been secretly working on for the past 4 years, I don't see a way they can move to a new engine without creating a ton of problems in the way the game plays. And as much as I want to see a graphics overhaul, I don't think I'd be willing to get it at the expense of the already stellar gameplay.