Honestly the only thing I want really fixed in terms of gameplay would be the morality system to Mass Effect 2 because it's an incomprehensible mess. It's not based on the meters you accumulate, rather it's based on the percentage of paragon and renegade choices you make throughout the game. The meters don't actually mean anything. This is why you can have an almost full meter but still not meet the speech checks for things like resolving your squadmates' arguments; it's because the invisible morality percentage isn't high enough. So if a speech check requires a 75% paragon score, it's literally based off whether 75% of the choices you made were paragon, not if the meter is 75% full. Nothing in the game informs you of this, and it's really fucking confusing, especially if you're going for mostly paragon but made a not-insignificant number of renegade decisions.
Mass Effect 2 is already guilty of forcing you to choose one moral path if you want the best results, but I don't think people realize just how bad it really is.