I have had some success with the +1 diplomatic bonus mission. The Reverend Mother can now execute this mission, and the AI will pick it. I called it "Force Friend". It kind of sounds like Jedi "Force" abilities, but I could not work "Voice" into the title.
I also made an interesting archeological find. There are two very powerful mission flags hidden inside the XML, which are implemented in the game but not used by any existing missions. One directly converts an enemy city to your civ, and the other throws an entire civ into anarchy for a given number of turns. The cost and difficulty can be set through the XML; presumably you would like to make them very expensive and also unlikely to succeed.
The "convert city" mission cost is not sensitive to any other factors, such as the city population size or whether you have much of your own culture there; I may try to add these.
I have added both of these missions as RM specific missions. We can play with the costs and success rates, but this will definitely make the RM powerful.
Next I will try to add a "Force Enemy" mission which gives a -1 diplo from the target civ to some third civ.
We had discussed before about two more possible diplomatic spy missions: Force Peace, and Force War. These would be identical to the existing diplomacy options, but with a variable success rate, and costing espionage points. It would be a good representation of the sneaky, manipulative Bene Gesserit if you could get civ A to declare war on civ B to keep A off your back. If I can figure out how the second picklist works for "Force Enemy", then Force Peace and War should be easy.
I am still not convinced that playing with *tile* culture is any fun. Although I see messages from IDW coming all the time, I have not seen any instance where IDW has any actual effect. It seems like a nice concept; but what actual difference does it make in any DW game?