While reading comments and other suggestions in "The Weakness of Expansionist Affinity and Possible Suggestions" thread, I was able to come up with a new border enforcement policies. (Special Thanks to @FlamingKetchup)

I suggest that the border enforcement policy should consist of 3 types:

-Open Border

    1) No grievance is generated when trespassing.

    2) Cannot declare Surprise War when there is at least one Unit inside the Open Border.

-Closed Border:

    1) Grievance is generated when caught trespassing. Thus, if no Units or Districts are present on the region to observe such trespasser, it is possible to sneak in without generating Grievance. This give you an actual reason to patrol your Border.

    2) Compensations can be demanded.

    3) Can attack trespasser without generating Grievance.

    4) Scout Units generate less Grievance.

    5) When Scout Units penetrate Closed Border and become caught, it triggers special Grievance that is unique to this: "Illegally Surveying".

    6) Illegally Surveying Demand is cheaper than normal Trespassing Demand, when compensating,

    7) The Grievance scales up with the size and total Strength of the trespassing Units and how many Turns they spent on foreign soils.

    8) Upon detection, both the region-controller and trespasser should receive some kind of pop-up message, which says that trespasser has been identified. And clicking on it should teleport Player's view to the sight/location of trespassing and highlight the trespassing Units.

    9) Upon detection, trespassing Units do not have to immediately retreat.

    10) No healing is possible when inside Closed Border. (Since supply is impossible)

    11) Units obtain negative Combat Strength bonus inside Closed Border. (maybe -2, -3, or -4?)

    12) Is a default diplomatic stance when meeting a new Civilization.

-Militarized Border

    1) No passage is allowed.

    2) Must declare War to cross the Border.

    3) Units with Stealth modifier, such as Saboteur, can sneak in.

    4) As soon as Militarized Border is signed, all remaining Units inside the Border must immediately move toward Player's territories, neutral territories, or territories under the effect of Open Border.

    5) Can be adopted only from Closed Border. Directly converting from Open Border to Militarized Border is impossible.

For coherent game-play, the already existing Allow Skirmish option would have to be separated from Border Policy. It should get its own section called Hostility on Neutral Area. It would have two options:

-Hostile Stance on Neutral Area

    1) can attack one another. But doing so generates Grievance.

    2) if the skirmish took place on 3rd party's territories (both attacker and defender have Open Border policies with the 3rd part), attacker receives another Grievance from the 3rd party, called "Caused Disruption on My Land"

    3) Caused Disruption on My Land Demand is cheaper than normal Attacked Me Demand.

    4) Is a default diplomatic stance, when meeting a new Civilization.

-Peaceful Stance on Neutral Area

    1) cannot attack one another. Period.

How do you like this suggestion? Your opinions are always welcome!