As I see it, there are two ways to approach any concept for GC3.
1) A pseudo realistic approach, whereby we justify the idea in real-world terms. I say "pseudo" because actual real world terms need to be somewhat flexible to even work in a game. For example, for all we know, there just isn't a way to go faster than light and therefore the entire game might not make sense.
2) Purely in game terms. Completely abstract value. In this way we suggest a feature would be "convenient" or "fun" or "needed to allow X". In general, method #1 is often taken too seriously, while method #2 whether we like it or not, is always a big factor, if the game is to actually function well.
Using these two methods I see pros and cons to the idea of "mines".
I will start with method #2, because it actually makes the best case for incorporating mines or "something" into the game.
1) Would it be fun? Sure. No one is likely to object strongly to a new toy. Furthermore, some players clearly like defensive gameplay.
2) What is the aim of the mines? To passively defend something. Should this be territorial borders or should this be discreet items like a planet or shipyard? Posts regarding territorial borders come up repeatedly and GC3 has a "vague" system of borders where they matter in some ways, and in other ways are completely irrelevant. Because they are not actually borders, they are actually influence, but the diplomacy calls them borders and get's confused by the matter.
I sadly don't see mines practically solving the territorial border issue people have. Several reasons, firstly, in order to defend a border with mines, the number of mines necessary would be enormous.... and I mean in game terms, we will deal with real life terms later. Think how many asteroid mining facilities you end up building... that is the most likely method for mines to be employed by the game, and it would result in a massive amount of new game objects. Their expense would have to be very minimal for them to be worthwhile, and then, their ability pretty damn impressive to convince a player to build so many objects. It's cumbersome and it wouldn't be fun on most maps.
So the only way to employ these rationally, would then to have a single object with a very large area of effect. That would work, but it would be another interface to consider, would still visually have to be represented, and the AI would have a big problem with path-finding through space.
So I do not believe there is a good way to implement the feature as "territory control". The border problem needs a different fix (namely representation on the map and diplomatic rules and AI changes.
3) This leaves us with using mines to defend discreet objects, such as planets or shipyards.
But lets ask ourselves something... Why? What is unique about a mine field that defends a planet or shipyard, versus say, 2-3 ships? Ultimately, it's a weapon that attacks ships, and presumably wouldn't be mobile.
How is that different than building a few medium hulls with a bunch of weapons and no engines? In game terms, it isn't. You could make a few tweaks though to give some uniqueness though. For example:
- Mines could reduce the health of the incoming attack fleet by 10%.
- Mines could use a separate move to attack them first, eating moves from the opponent fleet.
- Mines could be deployed only by a mine ship for a credit cost, or just be deployed on the map for credits, and not need shipyard production.
- Mines could have zero upkeep.
- Etc.
Ultimately though, these or other changes are going to be mostly illusory, if mines are made more powerful than ships to defend a planet you ruin the balance and could easily end up with a GC3 version of World War I... where defense has more power than offense. It would make the game pretty un-fun in my opinion.
This means mines must be cheap, but of relatively weak power.
4) What do mines accomplish that some other (and probably more realistic) defensive structure couldn't? Nothing in my opinion. Mines could easily be "Orbital defense grids" or we could look into rehabilitating Military Star Bases into actually defending planets. Orbital fortresses with some mobility make more sense in general... but now lets talk about psuedo realism.
Lets return to method #1, pseudo realism. Basically... do mines "make sense"? My answer to this is generally going to be "no".
1) Obviously, space is three dimensional. There is some conflict here, because GC is actually 2 dimensional... but in general, ships and other units don't ever "Have to" engage another unit because of space... you can almost always move around your target, you get attacked in space because you ran out of moves and another unit chased you down. Static defenses in this environment are extremely weak. You never need engage enemy starbases if you don't want to, and can move around them.
But real life has few or no examples of 3d mine fields... and GC3 is basically 2D, so lets just put aside the whole "I'll go above" idea.
How about in 2D? Are mines effective at guarding large territories? No. Generally armies, navies, have always found easy counters to mine fields on any strategic level. Vast mine fields between countries are seen to have pretty menial military value and have little deterrent to a determined force. Naval terms are probably the best analogy for us, and in this case mines never guard a strategic territory... the ocean is just too damn big.
For examples... why didn't the English manage to mine the English Channel or heck, the Atlantic ocean during WWII? Because its just too big an area. Mines were used to some good effect to guard discreet areas. Bays, ports, etc. could have mines deployed. For example the mines near Gallipoli of the Ottoman empire gave the British navy some trouble as they attempted to force the straits there.
So mines in space, MIGHT make some sense to defend a discreet area. Say a planet, shipyard, or other object.
But again... we are talking VAST areas. Space isn't like an ocean... it's inconceivably bigger, which is why there are so few games that try to realistically portray it, because they would be extremely hard for a player to fully grasp, and they likely wouldn't be very fun. The larger area you wish to protect, the more mines you need. All for what? The glory of not putting an engine on the damn thing?
2) Have static defenses worked well in a 2D environment? Yeah a little... but even then... consider the overall failure of castles to repel enemy armies during the medieval European age... consider the failure of the fortresses in Belgium during WWI, or the failure of the Maginot Line in France WWII, or the failure of basically any bunker complex today. Ultimately, static defenses are generally a loser's bet.
Just look at how heavy melee infantry fail against ranged cavalry units (like the battle of Carrhae or basically anything involving the Mongols) and you get an idea of just how sadly ineffectual static or slow forces are versus mobility.
3) What exactly do we mean by a "mine" in space anyways. Does it physically attach to it's target? Does it simply explode? Does it attack with a laser, or a gun? How does it keep position? Does the mine actually only work once?
If you actually start trying to design a mine in space, you quickly end up with a semi-mobile satellite with a ranged weapon, because space is so damn big and being out-ranged is so easy, because space is also incredibly open. If the mine is "low range" you suddenly need more of them to cover the same area as a lesser number of long ranged mines. So pretty quickly we begin losing the identity of a minefield, and would more accurately have a "Defense Grid".
A defense grid would at least have the advantage of doing damage without necessarily committing suicide, so the return on investment might be more than a single battle, even if you are defeated there would be salvage... if your "space mines" actually explode there would never be any salvage to recover and you would lose at least one mine to even the tiniest of enemy units. Whereas a defense grid might happily vaporize the tin can that is thrown at it and waste nothing.
4) So minefields are A) very unlikely to work well in three dimensional space, 
would be massively expensive, C) would have little ROI because they would literally be a one-use weapon, D) by the time you overcame technical issues, you end up with a series of missile launchers, drone fighters, or defense Satellites, not a traditional mine field... E) even if we ignore a lot of these issues, they weren't even very effective in real life in a 2D environment...
So my conclusion is this... mines don't tend to exist in space games for good reason... from a realism perspective they are very unrealistic, and from a game's abstract perspective, they simply don't need to exist, as other tools already exist to defend you, and are generally more efficient. Can mines be fun? Certainly. Do I think we should add them to GC? Sure... I'd call them a "Orbital Defense Grid" make it a object build around a planet to add to its defense, and call it a day.
Biggest problem though, is that it will not solve the "my borders mean nothing" problem in the game. To solve that, we need to A) Define borders, but then separately define "Influence" for culture. Teach the AI to care about borders, and C) have a diplomatic system that works with borders.
The game completely works without these things though... it's just psychologically unsatisfying that the AI constantly ignores your over-blown borders (influence), that you've formed a sentimental attachment to. In GC currently, control of territory depends on your ability to guard it with fleets, and your willingness to commit to war with everyone. It's not what I would have designed but it does function and allows for the AI to grab planets and resources you are not working hard enough to grab yourself.
