Well I would like to start this off by saying I love the mod (currently running Floris, which has it built in, as all mods should) And I have a bit of a larger idea then you may be used to over here. Something that would be an almost full-on expansion to the mod.
The concept?
Diplomacy-Power Behind The Throne
In essense the concept is that good kings rule the traditional way that Diplomacy has near-perfectly presented, but great kings rule from the shadows, behind the puppet that sits upon the throne.
Currently the best a player can do is buddy up to a king and his lords and hope that they hang on his words enough afterword that he/she gets awarded extra fiefs or even marshalship, but this is not sufficient for the more cunning player, and thus I propose a myriad of additions.
These are not comprehensive, but they would make a good start for this new approach to Diplomacy:
1. First one is fairly crude, but it will do for now: A "influence" bar with each faction, indicating a players overall infiltration of the peasantry, guards, and other rabble. Its purpose is to allow players to accumulate such abilities as having guards free the player from prison (or even run their shadow-empire from within the jail itself), walk straight into enemy towns with no fuss, and easily sway captured faction soldiers to join you after capture (perhaps with no morale penalty?)
I would suggest something like [Relation with faction villages+towns*Renown+Honour/Faction Relations] for the algorithm of what your influence is (thats probably terrible and imprecise for the maths, but its something to work with anyway)
2. Recruitment of spies/agents among the peasantry who gather information and execute your will within the target faction. (would probably need to include these in the "influence" algorithm somehow but I'm just brainstorming) These would be available regardless of whether you are currently fresh off the boat, a faction vassal, or a king/queen yourself (although influence would determine their availability and efficacy) The spying part is obvious but agents would also enact missions such as recruiting other agents, bribing guards to increase influence, and poisoning nobles to reduce their ability to lead men and such. The agents would likely require some form of leveling system, possibly based around Persuasion and the Agility tied skills? (once again, brainstorming)
3. Possibility of creating forged/false orders to both lords (generally for military directions) and higher ranking peasantry (tampering with the nations Constable, Chancellor, and Treasurer
)
4. Finally the possibility to eventually completely supercede certain lords or even the king, this would keep said character as a "front" while transfering their powers upon the player. (not sure how this would be accomplished, but once again BRAINSTORMING!)
And those are the basic concepts for the start of Power Behind The Throne. Anyone who wants to chime in/shoot down my hopes and dreams feel free!