From GoaFan's 4x mod...
Improved Diplomacy and Espionage
In an attempt to make Diplomacy easier to pursue, the functions of Vanilla envoy cruisers have been given to a new entity called embassies. Embassies are built by envoy cruisers around planets owned by other player and cost no fleet supply, potentially allowing one envoy cruiser to build enough embassies to max out your relations. The embassies themselves have the former abilities of envoy cruisers, but do not have diplomatic immunity, so you should only build them on friendly planets. Also to make up for embassies being easier to deploy and maintain, the max relationship bonus per planet has been reduced to +1, so you will need to have more embassies than vanilla envoys.
To give envoy cruisers another use after building enough embassies , they have been given 3 new espionage abilities. One shared among all three is the ability to deploy a phase probe, which is basically a mine modified to spy on the gravity well it is deployed in instead of explode. Like other mines it can only be removed by scouts once set up, and are a great way to get intelligence on distracted players. They will only work in neutral or hostile gravity wells however, to prevent players from laying them in friendly gravity wells of players they intend to backstab.
The other two abilities are unique for each faction, but they are all used against enemy planets, with one lesser ability that can be used a safe distance away from the planet and one greater ability that needs to be used right next to the planet. Their effects range from simple trade penalties to completely halting population growth, and last for several minutes, though the cool downs are even longer to prevent them from being used on multiple planets. Also the diplomatic immunity ability has been mortified to prevent envoys from using other abilities while invulnerable, so its best used to allow your envoys to escape after using the abilities, not for breaking into fortified planets.
While the Sins AI is not very moddable, a few modders, most notably ZombieRus5, have discovered a few tricks that can improve their performance. A few of them have been included in this mod.
The advent homing mine squadron has been removed from advent carrier cruisers (the AI would often just build many squadrons of mines, then bring them into combat without deploying them, making carriers a dead weight on its fleet supply and economy). Instead, the carrier can deploy homing mines with an ability like the Vasari minelayer. While this means you can no longer deploy squads from across the gravity well, the ability to keep your strikecraft deployed at all times should more than make up for it.
Each faction's AI has been given certain techs it now prefers to research. The AI is quick to get research that unlocks ships or abilities, but other abilities are mostly ignored unless it has so many resources it doesn't have anything else to use them for. While the AI will still prefer to max out its available fleet supply than research, it will now get certain techs, like Phase Missiles for the Vasari or Armor for the TEC, much quicker than normal.
Have we ever considered creating a new frigate or cruiser to perform the espionage abilities and add that research to the diplo tree?
Have we incorporated any of the AI ideas here? Advent homing squadrons (when used by the AI) do suck after all.