tl;dr: Taking the 'losing is fun' mentality of Dwarf Fortress to the 4X genre, and building in mechanisms for wholesale collapse of civilizations and their infrastructures ala the Fall of Rome and the Dark Ages. Because some people (like me) just want to watch the world burn.
You know one thing that always bothered me about games like Civilization and Age of Empires and basically every single 4X game ever? The technological advancement is always linear and upwards, as if it were impossible to lose/forget developments and every single building in your entire nation had psychic access to an indestructible library of all the tech information that your nation has researched so far. Even worse, the technology ends up being /free/. Tech research is nearly universally a bonus with no downsides, giving you more and more options and more and more production at the expense of... nothing, really.
Suffice it to say, reality doesn't work that way. If it did, we'd have had factories of mass production in the iron age. In real life, technological applications tend to come with increased costs in consumed resources; building a skyscraper in the modern age requires far more infrastructural support and resources of all kinds as compared to a hut of the stone age- an incredibly high factor of increase that modern society only manages due to equivalently high factors of increase in the resource-producing sectors. Society is an interconnected web, and every single part relies on every other part.
What this means is that if any one part malfunctions, the whole system is liable to come crashing down.
If, in real life, the generators of any city failed, the power shortage would cause most if not all industry to stop until the problem was fixed. If it weren't fixed, technologies that require electrical power would be effectively lost in that city; people couldn't rely on computers or air-conditioning or refrigeration, anymore. That then creates a whole new host of problems; for one, the city's food distribution would basically be crippled as all the previously refrigerated food uniformly went bad, causing strife and perhaps starvation of its citizens...
We have backup systems and redundancy just in case of these events happening in real life, which is why power shortages are not considered apocalyptic heralds and rather a passing night's inconvenience. What I am rather getting at here is that there is /no such thing in most 4x games/, as far as I know. Resource pools are nationally based, instead of regionally based; if the enemy captures one farm, that doesn't really change much when you've still got dozens of other farms supplying your empire with food. There's no interconnectedness, in other words; your empire's woodcutters and miners will function just as well plus or minus one measly farm somewhere. The fascinating interaction between economic subsystems is virtually unheard of, due to the simplification of technology into a rising slope with no additional incurred costs and the simplification of economies of scale into a big national-amalgam with no consideration for subsystems.[/spoiler]
So let's get onto some game mechanics.
First, the whole paradigm of the 'tech-tree' is done away with. Instead of the entire nation as a whole having a single large tech tree starting from the stone age and proceeding to the modern and future age, every single type of structure and every unit has its own tech tree. Most of the upgrades, in addition, will not be free. Rather, they will incurr an 'upkeep' where resources are consumed every time-unit or turn (depending on whether this is in a TBS or RTS format), in exchange for the bonuses that the technological improvement provides.
As an example of the concept of impermanence-of-technology, you could choose to upgrade your stone-age troops with +5 damage Bronze Spears, but spears get damaged or lost over time, and you'd have to feed the troops an upkeep of 1 Metal every turn, or lose the +5 damage bonus. Knowing what Bronze Spears are and how to use them is not the same as having a Bronze Spear, after all. A further advancement, perhaps, might be an Iron Spear upgrade, which raises the upkeep per turn to 5 Metal per turn. As you scale the tech tree, upgrades rapidly increase in cost, forcing you to develop your resource-harvesting facilities in parallel- raising the resources that each facility consumes in order to produce enough to feed into the upkeep- creating something of a spiral that perpetually forces itself upwards, always hungry for more resources to feed the ballooning upkeep that comes with technological development.
This also introduces an interesting new gameplay element- the usage of /shortages/ as viable mechanisms of conquest. In most Civ games, losing one farm doesn't change much if you have a lot more. However, if you lose one farm in this game model, you'd not only lose the food production, but any citizens being fed by that food, which would chain into a massive infrastructure collapse as mining and woodcutting and all sorts of harvesting failed in response to lack of manpower. Sometimes, chaotic events of nature might even facilitate such an unexpected- but frequent in real life- infrastructure collapse.
For example, imagine the following situation. A pair of farms has gone quite far up the farm tech-tree, and have upgraded from a +20 production: Irrigation Techniques (-5 water per turn) to a +100 production: Surplus Irrigation Techniques (-30 water per turn). This huge increased cost in water is counterbalanced by the research of a new technology, the Pump Well, which offers +60 water per turn as long as there's an Engineer on the tile- as opposed to the old Manual Hauling method which only gave a measly +15 water per turn. As a side effect, where in the previous model the city could store up 5 surplus water per turn in warehouses, here the city cannot store any up- but that's okay, right? Tons of excess food is being produced! All this excess food can be used to fuel civil rights policy upgrades to increase quality of life for the citizens, and the excess food will allow us to field more citizens to put to work in mining and forestry and the military. The empire looks to be on the verge of a golden age!
And then a plague hits our city. Half the population of the city randomly dies- including our Engineer manning the pump.
The city's university can only produce an Engineer once every 5 turns. In the span of these 5 turns, the two farms find that 15 water per turn can't even fuel a SINGLE one of the Advanced Irrigation Method technologies. Both farms downgrade their technology back to the simple Irrigation Method, reducing food production from 100 to 20 for the next five turns. The loss of 80% of its food production rather quickly empties the food stores of the city. Citizens start starving and dying even after the plague has already passed, a cruel one-two punch to a once prosperous empire. The militaries and mines and foresters, of course, probably also have to downgrade their technology a few levels or so after losing the ability to upkeep whatever food-related technologies they'd researched- maybe some sort of elite diet for the soldiers that turns them into fearsome sumo warriors or something like that.
On the bright side, over the course of the five turns, the city accumulates a surplus of 25 water. Should be useful in case the next disaster to hit the city is an Urban Fire that consumes surplus water before it consumes the HP of buildings in the near area.
Oh, and your spies report that the plague was in fact delivered to our city by an agent of a rival nation posing as a simple merchant. While we were recuperating from the collapse, /they/ were building up an even more advanced power base with the capability to, among other things, produce 250 food per farm per turn (at the cost of 100 water, 20 metal, and 15 stone per turn)! We'd better catch up fast. Maybe if we crippled one of /their/ new super-farms by raiding a stone quarry or a mine to stop their supply of stone and metal...
I've got a lot of other stuff to say about this idea of mine, but I think I've typed up enough of a textwall as it is. Any thoughts on the idea?