Shake up the formula once, shake it up again, and shake it up some more. You can clearly tell that with the isometric viewpoint reminiscent of a number of games I played that things are gonna try to look a little more 3D. There's no grid-based movement, but arguably the true big change was actually putting in the heroes themselves to be battlers alongside their troops. This shakes up the formula quite a lot, now you gotta worry about keeping heroes alive and not to fall in battle, alongside all the other familiar things such as your towns and resources. The regular navigation is also 3D, and there's quite a lot to go through for both the main game and the expansions.
They also shook up the overall storyline of the whole universe. The world you know from previous Heroes games gets nuked, all cause Tarnum didn't really make it in Chronicles to stop Gelu and Kilgor from crossing those two legendary blades, destroying all of Enroth. Sure, Tarnum lives again, as do a number of others who, by some stroke of luck, manage to enter portals leading to Axeoth, the new world. But Heroes 4 doesn't really provide us with familiar faces to play with this time around. Everyone is new. Everyone has individual campaigns, ranging from love stories to conquering oceans to just being a barbarian son of Tarnum. The backstories tend to be far more interesting than the individual scenarios.
This does change with the Gathering Storm and Winds of War expansions. I don't know what it is, but I adore it when characters come together. Both expansions do this but in different ways. In the Gathering Storm, you build up the five main characters and they all join together to battle and defeat a greater enemy. In Winds of War, you build up the five main characters and have to have them battle each other while conquering a foe they all are trying to conquer. The latter expansion feels like you should play the most villainous of villains, cause the last scenario, you pick one to be your main conqueror. It does interest me how Heroes 4 isn't as well-received as the previous installments. I didn't find it too bad, but the shakeup of the formulas does play a part.
