Edit; Before you read the rest, I'd like to hear your opinion from a gameplay point of view, although realism considerations are valid also.
Just throwing out an idea; what if resources, be it in the form of asteroids, minerals on planets or galactic resources (amoung other possibilities) were able to be exhausted? Different sources of resources could have varying amounts of them to start, while building things which take advantage of them would use up a certain amount per turn. This would eventually lead to depletion, which would cause new resources to spawn elsewhere, mandating that you either seize new resources by force from others, or harvesting them yourself through a "second colony rush" (Or negotiate for it, but pshaw to that.) I can see this being applied to much of the new, anticipated "Galactic topography" that's been hinted at as well.
For instance, a planet would only have a limited amount of raw material needed to manufacture things, asteroid fields would be destroyed by space miners, gas nebulae would lose their helium and hydrogen etc... The point I'm driving at is that this idea has no constraints, and can be applied to as many or as few things as possible.
A post explains this very adequately in terms of realism:
You dont have to 'run' out of physical ore or energy source or whatever your extracting for it to become uneconomical to continue extraction efforts.
Since there is clearly a lack of understanding of the basics here. Humans, and (aliens) always go after the 'low-hanging' fruit first. The easiest (cheapest) and highest-quality reserves will be exploited first.
That would be on planets with breathable atmospheres.
Next to go, would be marginal planets with poor atmospheres, moons, high gravity whatever.
After that-closest metal rich asteroids
Then-you go for most distant and miserable asteroids and hunks of rock in your solar system.
Once those go marginal(even more so than they were al-ready), you start to look at transporting raw materials over inter-stellar distances. Once you get to this point, the final cost of a ton of refined metal will be literally, astronomical to end-users.
Anyone that thinks it is 'impossible' to deplete non-renewable resources clearly has zero eduction in the matter. Once a ton of metal costs more to mine, refine and sell to end-users than the economy can afford to pay, the resource is 'depleted' for all intents. Even at that point, there will be 'lots' of metal still in the ground, or in asteroids or whatever, but they will 'stranded resources'. The definition of which is resources that are too diffuse, too low in quality, or take far too much energy to extract and refine profitably, and bring to market.
A game like Galciv could fairly easily model resource depletion. It would be certainly make things interesting as stellar empires with vast fleets and trade networks and 100s of billions or citizens would be consume non-renewable resources at ferocious rates. Very few 4x games tie resource DEPLETION into there models. I think this is because a lot of us assume (incorrectly) that resources are either infinite, or that technology and say secondary efforts like recycling can extend resources infinitely.
None of the assumptions have any basis in truth. The only game I can think off the top of my head that featured resource depletion, if in a limited way, was Homeworld. Asteroids were mined for materials to build ships with-and they DID deplete so you not only had to go out-harvest, guard the harvesters themselves, but no matter how good your 'tech' was, you *would* run out of resources eventually.
Here's an opposing viewpoint:
I disagree with this. Presumably, even marginal worlds have gravities similar to the gravities of inhabitable worlds, yet since they are marginal , you wouldn't expect heavy industry to be present. Therefore, you'd need to get the ores off of these marginal worlds in order to make good use of them - and getting stuff off of a planet with a gravitational attraction similar to that of Earth is expensive. The stuff located on moons of developed planets and the stuff located in nearby asteroids is far more easily accessible because it takes much less energy to move to the point of use. Moreover, depending on the actual atmospheric conditions, it may actually be cheaper to set up mining operations in hard vacuum than on one of these marginal worlds with poor atmospheres - just look at Venus. It's much harder to deal with atmospheric pressures tens of times greater than normal in conjunction with temperatures in excess of 700K than it is to deal with low temperature conditions with poor heat transfer and little to no gravitational attraction.
The ideal location to go after you deplete your most easily available sources of a resource are those areas which allow you to most cheaply move the raw material to its point of use. That is emphatically not a completely different planet, especially if that planet has a gravitational attraction sufficiently similar to the homeworld for it to be habitable (and the higher the gravity, the worse the planet is as a mining prospect). By the time you're looking at other planets as sources of raw materials for your industry, you may as well be relocating your industry to the planets that have the materials, because it's very unlikely to be economically feasible to get the stuff moving at escape velocity or higher, move it to the world which represents the point of use, bring the stuff down to the planet's surface, turn it into whatever you're trying to make, and put it back into space again. Moving something that's already in space over to the point of use cuts out one of the three surface-to-space transitions, which are likely to be the most expensive stage of transporting the materials.
The asteroid belt in this system, according to Wikipedia, is estimated to have about 1 million asteroids of diameter greater than 1 km. The volume of an asteroid modeled as a sphere of 1km diameter is a little over 500 million cubic meters. If we assume that the largest classes of GCII ships can be modeled as a rectangular box of dimensions 1kmX0.5kmX0.5km and are 80% hollow, then just one such asteroid has enough material for at least one such ship given that the volume of usable material produced from the asteroid is equal to 10% or more of the asteroid's volume.
Thus, if we assume that only 10% of the asteroids of diameter greater than 1km are useful as sources of ore, and that only 1 ship of the largest class can be built using the materials obtained from a single 1km diameter asteroid, our solar system has enough material for 100,000 such ships. I don't know how your games go, but 100,000 ships of the largest size category seems to be significantly in excess of the total number of ships I build in a game, and I would say that it's several orders of magnitude in excess of the number of ships of the largest category that I build in a game.
Even though GCII has enormously fast production times on all sorts of things, it doesn't provide you with the economy to sustain that rate of production for a sufficiently significant time period for it to be reasonable to even consider that you might deplete a single system's asteroid belt (our own), nor does it provide you with the economy to make it feasible to consider having that much stuff in service at any point in time.
Additionally, according to Wikipedia the global annual production of steel is ~1.3 billion tons. Assuming that the tons referenced here are short tons and that the density of steel is 8050 kg/m^3 (the high end of the density range listed on Wikipedia), the volume of steel produced annually on Earth is about 150 million cubic meters - enough new steel to produce three ships of the largest size category per year, as modeled earlier (I will grant that not all of that would be suitable for use on starships, nor would all of it be available for such, but this is nevertheless an indication of the current capacity with just a single planet's resources).
Yes, it's reasonable that eventually you'd run out of resources in certain areas. It may even be reasonable that you could eventually deplete a star system of its economically viable resources. However, on the time-scale of GCII, and given its production levels, it is unreasonable that you could deplete planets and asteroid belts in a time frame which can impact the game, unless GCIII turns represent years and models actual mines rather than mining operations that make use of large asteroid fields.
There you have it.
That being shown, don't forget that gameplay is typically king over realism, so you should approach this as a question of taste and opinion, not of practical reality. What do you think about this? Is there anything a Stardock employee can hint at?