We have made a ton of improvements to the game as a whole! If you just want to stay current then you should purchase the second edition update kit. Which contains updated rulebooks, buildings boards, commanders, cubes, dice and 2 new maps!!
However, there have been improvements made within the other expansions, "Solo/Coop&Fog of War" and "Lost brigades" that are worth checking out. A new terrain pack with mage structures is always nice to have.