ZoC should work as 1).

Within the scope of the game, i think a ZoC is defined as the space in which you can deploy starbases and military ships without asking anyone => therefore borders as in political borders.

In the ZoC of another empire, you should not be able to move in military ships, deploy starbases or even claim strategic resources or colonize worlds. In fact, even unarmed ships like colony ships and scouts should not be able to cross an empire's borders without permission.

And all this is a good thing since it brings more diplomatic options on the board, treaties like open borders for civilians ships (means unarmed), trading rights, mining rights, colonization rights, grant military access, establish military starbase and maybe more.

ZoC should be defined by colonies only.

Influence sphere, as ParagonRenegade said, should be separate and should represent how much Coca and chewing-gum guys on the other side of the political border drink and chew.

Spreading influence maybe a problem if as said before, influence starbases can only be build within ZoC but it makes sense, since i don't think ex-USSR would have authorized USA to open a USA Influence Consortium in Moscow or USA authorized ex-USSR to open a Communism Propaganda Center in Washington.

I think that before making an agressive influence conquest, trade and good diplomatic relationships must be established since with good diplomatic relationships, trade flourish and with trade comes influence. Good relationships can enable the creation of a influence starbase in the other empire's ZoC, therefore cultural conquest.

Espionnage could be used to spread influence as well, with backroom talks and similar subversive operations.

With territorial proximity, influence provided by an influence starbase could just cross the political border.

However, i must say that i don't think colonies should just switch from on side to another. It doesn't work like that in real life. Colonies should switch only if claimed by an rival empire (which is most likely a friend since it has influence based on trade, good relationships, see above) or if they ask for switching sides and are accepted by the rival empire.

But a colony switching sides should always ends up with a great diplomacy penalty (of course the losing empire isn't going to be happy to see its citizens tax leaving).

Reading this might make you think that military conquest would be easier and therefore preferred by players, which is not a good thing because having different, equals ways of conquest is a good thing.

Well i agree, but i don't think influence conquest should be simplified, it's military conquest that should be harder, especially in the field of keeping the military conquered colonies under control: there should be riots, sabotages and generally speaking, unwillingness to obey the new ruler.

It should translate in long-time maluses in taxes, production and research provided by the colony slowly decreasing over time (maluses even higher than 100%, meaning that an angry colony should costs to maintain instead of providing anything). A revolution should even bring the colony back to the original owner if there is not enough military forces to keep the system under check.

well i'll stop here since i probably went a bit far away from the question and that the game is most likely too much developed to include all this but anyways, since i write all this, i should as well post it!

I go for option 2.  Your political borders start and stop with your planets.  There are no other physical borders possible.  You could possibly claim to enforce political borders in space, but it would be more expensive in border patrols than it would be worth.  You end up with the same general unenforceable "my claim is better than your claim" chaos that defines current air- and sea-based border claims.  (Then again, it can get immediately enforced upon declaration of war.  That is an inconsistency I cannot reason or imagine my way around.)  

Your influence is a calculated combination of factors based on economic and cultural pressures.   How you exude cultural influence over vast regions of empty space is a little vague, but generally seems to indicate that if some errant space craft was passing through, this is whose subspace radio stations and talk shows they would be listening to.  Makes sense to me, anyway.

And of course, a good military invasion trumps everything.

i could vote for this 

i think culture can influence borders but they should be separate systems

This is interesting and I had never seen it or never noticed it before. If this indeed is Stardock's vision of how borders work in GalCiv, I think they should make it more explicit, as the way area's of influence are depicted on the game screen strongly suggests territorial borders on a political map.

I see them more as cultural borders, in which one has political influence.  One issue that Galactic Civ suffers is that the genre norm is fixed borders any intrusion must be seen as an act of war and thus the opposing player entering your borders must declare war first or have a treaty otherwise they cannot enter your lands.

This is no where near the way things should be done. If someone with an army enters your land, you can declare war on them, they do not need to declare war first. In space, if you enter my territory and I say, hey get out... what tools do I have to force them out... They don't have to listen without the threat or use of force. If you attack their vessels no matter where they are, that civilization will see that as an act of war. In fact, I should be able to attack their vessels without declaring war, it should be the responsibility of the opponent to declare war on me at that point.

It is a matter of perceptions. If I sent a vessel in the middle of hostile territory and they attack it, I don't necessarily want to declare war or be at war with that nation. I can choose to ignore the attack. This would change the mechanics of the game and bring diplomacy to a whole new level of discomfort. The idea of the "gray war", a war that is not declared by either party although past actions indicate otherwise.

I do not support this full change to Galactic Civilization as it would change the flavor of the game, although some aspects of this diplomacy would be nice to see.

I am very much opposed to using influence boundaries as territorial borders. If Stardock wants to add in 'national space' as a game concept, then I'd rather that it were some fixed number of tiles around a colony rather than anything remotely similar to influence, with a reasonable rule for settling overlapped claims. Conceptually speaking, there's little difference between 'territorial waters' and 'territorial space' - both represent a region in which the laws of the owning nation are generally accepted to apply, rather than a fully-enforceable border. It also represents a limit to how close foreign warships can come to a colony without causing a diplomatic incident during peacetime, and would be an actually-reasonable region of space where you might have to pay a starbase tax.

That's just the point. If you look at GC2 your influence can extend into sectors where other empires also have influence, making the "boarders" rather murky. In GC2 the sectors that get your empire's color are those where you have at least some influence and more influence than anyone else.

These are NOT territorial borders.

^It seems one of the only ways to do borders is via culture. If we had to claim every sector it would be a drag.

I suggest you take a look at the DiplomaticModifiers.xml. Several of the modifiers seem to indicate, that the devs intend to equate cultural borders with political borders.

The best comparison to borders in space would be borders at sea. At sea most nations have a 12NM (22,2km) zone that is the nations territorial waters (TTW). State vessels/aircraft from other nations must request diplomatic clearance in advance in order to lawfully enter within the 12NM border. Some nations - often poor nations)chose to have only a 3 NM TTW for various reasons. If 2 or more nations TTW borders would overlap the principle  of dividing in the middle is applied. Exceptions exist for straits where all ships have the right to pass through but not to conduct any kind of operations.

In addition to the TTW nation have an Economic Exclusion Zone  (EEZ) extending 200NM (370km) from the coastline. In that zone the freedom of the seas apply, in that all ships are allowed to pass through without requesting permission from anybody. The nation which has the EEZ is the only one allowed to exploit the natural resources in the water, on the seabed or in the underground - eg. fish, oil etc.

I think such a principle could be applied to GALCIV 3 where TTW (or TTS) extends a fixed distance from settled planets or claimed systems - other species should be able to enter this volume without permission - to do so would be an act of war.

Movement should be unrestricted in the EES but construction of space stations or other use of resources (perhaps even using a survey ship to look for goodies) should be prohibited. Violation could cause a hit in standing, a claim of reperations or constitute a Casus Balli - enabling the wronged nation and its allies to go to war against you - and leave you in a situation where your allies would not want to help you.

It boils down to the military border you establish is only applicable if you are willing to fight for it. Thus, if the other nations intrude on this space, it is you who should declare war. They should abide by your set boundaries yes, and if you say that is an act of war for the entrance of those ships, then declare war on them.

The only natural borders in space are the planets themselves / any place you put a space station. These are the only areas which you have a physical presence and if you cannot enforce that border, there should be no reason why the "enemy" shouldn't be able to utilize this without a political declaration of war by the invading party.

I would like to see the war mechanics in a game change. The invasion of a planet is by all means a full declaration of war by the invading party. To physically remove your elements from your well established area cannot be anything, but be a declaration of war. The building of star-bases and maneuvering without conflict, follows closer to a "Cold War" situation than anything. This is similar to how the game runs with influence, a Cold War fight in general.

If the diplomacy in the game starts to adapt, where one can threaten war or political retribution against other parties, it will be impossible to force the modern society national territorial waters. The only reason to respect these boundaries is that they have full control and power to defend these waters against invaders or aggressors. The political community of the world tries to ensure that conflict doesn't arise out of such disputes. A similar implementation is required to force such a fluid border that we have in Gal Civ. (But we have to wait to see what diplomatic elements that we can have in the game.)

Yes, this would be an act of war, so diplomatically if war was undeclared it would (should) be declared by the aggressor.