I'm gonna echo some things already said:
Firstly: Up the difficulty. A lot. The base difficulty is bloody mindless. This was a problem with Bionic Dues as well. I seem to remember complaining about this back during development of this game. For BOTH games, that issue pushed some people away, which is a real shame.
Second: Er... you really don't have to do that much more to make it more interesting. When the difficulty is higher it FORCES a lot of chaos on you. More bad things will happen, more bandits will show up, your score needs to be ever higher to progress from one phase to the next, and all in all, things will get more than a bit wonky. It becomes easy to end up in a situation that's rather out of control.
Also, as Logorouge says, you HAVE to keep the violence and chaos happening in order to win. You must achieve a certain score before the end of each "age", or it's game-over. This is only done through actions that inherently cause more chaos. Units dying and buildings (that are yours) being destroyed are the most basic things that increase your score. If you're just sitting back and letting your guys very slowly punch each other equally, you're not going to win. At least, not if you upped the difficulty as suggested. Only by unbalancing things can you manage a high enough score. For instance, if your buildings ARENT getting smashed up, you're not doing it right. Each side should be doing significant amounts of city smashing. Keeping that under control is part of the challenge, but it's absolutely necessary. Same for dealing with high-power, rampaging units. You have those mythical units for a reason; they're worth a lot when they go down, but they tend to do an inordinate amount of damage to basic units on the enemy side. Placing them strategically (since they can be placed directly, unlike normal units) is important.
So, yeah, again, keeping things balanced is actually the direct wrong approach. The whole concept of the game is that you're not only keeping this war going, but you're TILTING the balance back and forth, but trying to keep it from going TOO far before tilting it back.