Civilizational Traits
While giving the "Citizens" thread a once-over and the short discussion on peasantry defense, I thought to myself that "Full peasant defense would be ridiculous, most of these untrained and unworthy plebes are hardly fit to carry a rake, let alone have any loyalty to the Primarchs, God and the Empire or the courage to stand stedfast in the face of assured destruction". At the same time, I'd like for my peasantry to be.. well.. better. If nothing else, I'd like them to stand there and take a beating, even if they're dying in droves. Cowardice is, after all, a cardinal sin.
Obviously I can't say "The idea of full peasant defense is stupid, but I'd love it myself", so enter traits. 
Most should be familiar with the concept unless you suck. It's a common staple in a variety of roleplaying games where you choose one or a few traits (or none) for the character you create. All traits have positive and negative influences on your character. The obvious difference for what I'm suggesting is that they'd be for the entire civilization that you create and that there'd be no limit on the number of traits chosen (some would, however, be mutually exclusive). In-game, traits would be hidden for opponents.
Assuming that civilization-creation is analogous to that of Galactic Civilizations 2, they'd cost nothing to choose and be entirely optional. In many cases, it could simply be a matter of roleplay, perhaps opening up additional choices in the game. Some could add buildings or technologies later while excluding others permanently. Some could possibly be extreme, excluding an entire Element while boosting a seemingly arbitrary stat - others could be as minor as doubling the construction speed of farms while halving that of barracks.
Apart from obviously discussing this, by all means, come up with some interesting suggestions. Try to keep it general, since obviously we know nothing of balance at this point.
Allows all untrained citizens of an attacked city to participate in the defence of said city. However, should the enemy be repelled, losses on part of citizenry will still be much higher than usual.