Moderator Action: If you want to talk about features, fixes, or changes in the next patch, here's the thread to do it.
Ok, here's 2 suggestions...
First suggestion: Countries A, B, and C. C has open borders with B. C attacks A, going through and attacking from B territory.
Currently, B takes no diplomatic hit/etc. towards A. It seems unrealistic that B can allow C to use its territory to attack A without B being held in some way accountable.
B should be attackable by A without A declaring war (and without A taking the penalties for declaring on B) - for technically B declared on A when B allowed C to use B's territory to attack A.
This would be a big change in how wars are prosecuted. No longer could countries march blithely through a number of intermediary countries to make attacks.
One way to reduce this big change is...
Second suggestion: there should be 2 different 'open border' treaties.
The first is a low-level treaty that allows passage to non-military only (great people, missionaries, workers, etc.).
The second would be a higher level treaty, on par with a defense pact treaty, that would allow military units passage. This second treaty would incur the penalties/responsibilities mentioned in my first point - a country that allows passage of military units that are used to attack a third country makes the 'allowing passage' country accountable.
This change could be handled by adding an 'alliance' type of option.
Real life isn't always a good yardstick, but in this case I argue it is - consider what would happen if Canada marched troops into the USA, then used them to attack Mexico. This sort of 'open borders' are only possible with alliances such as NATO, or a similar type of treaty.
*thumbs up*