First of all, the fixes from version 4 (or rather, its bug fixes; the improvements could certainly have been adopted). An updated approach to both development and combat. More visual appeal, a more complex combat system (in my opinion, version 4, and even more so version 3, is too predictable, making it resemble chess where you need to think several moves ahead). Combat should be unpredictable; there should be a unique feature that, when used, can drastically change the situation on the battlefield. Otherwise, everything boils down to experience rather than ingenuity.