Militias and Levies
One of the things I'd like to see most in Elemental is some kind of town militia for settlements. These are not unit types that you recruit an keep around indefinately at the cost of some upkeep, as is typical in many games. Instead, they are citizens that automatically muster when a settlement is attacked by enemies and swell the ranks of any defenders that already exist (if any). The numbers and quality of militia that rally to defend can hinge on numerous factors. First, wealth and size of a city will be one factor, as well as any special militia training buildings. Also, the popularity of your rule, as well as the popularity or oppressiveness of the invader, can influence size of the milita force as well (no one will want to fight to defend your sovereignty over them if you are a tyrant and the invader is benevolent.)
This element will give defenders a distinct defensive (and historical) advantage, as well as prevent single units of swordsmen from capturing massive megapoles that you may have forgotten to post soldiers in.
I also have a similar idea: Conscripts. If you are planning a powerful offensive campaign or a concerted defensive one, you could proclaim a call to arms, which would greatly reduce productivity, popularity, and numbers amongst your citizens but swell your ranks with temporary soldiers. Afterwards, you can dismiss them back to their homes, where productivity and popularity begin to recover (unless too many of them are killed in action!) Longer campaigns with such conscripts would begin to stack the above mentioned undesireable effects until they would be unbearable, and force an end to the campaign or force peace talks.
If Elemental focused on heavily on conscripts for all out war, it would help prevent the "steamroll" effect that plagues so many other 4X games.