Okay, I'm going to do a better pitch for adding Diplomacy to the game as the better the pitch the more likely you are to implement it. Just saying implement a mix between MOO3 and Balance Of Power II may not be the most effective strategy.
Okay, there are six other empires in the galaxy. The green brigade and the yellow legionarres which are at war with each other. The purple agriculturalists. The orange scientists. The Gray Industrialists. The Pink Artisans.
These empires have no planets visible on the galaxy map because they are just too far away. In story terms, you have to jump through worm holes to get to each of the five sectors of the galaxy(green and yellow share the same sector hence why they are at war). So, trade and long term communication is possible but any sort of warfare is not practical.
The reason why I don't think there should be more than a 1v1 war is because adding more fronts makes the game take longer. Whereas, this way you get all the diplomatic benefits but not the downside of having a super empire and having to spend several turns mopping up the smaller empires.
Each empire has a relationship with all the other empires between -100 and 100.
Every 10 years of game time there is an intergalactic senate meeting where various bills can be passed. The number of votes everyone has depends on the population.
Bills can be passed like cease fire between blood legion and blue army, cease fire between green and yellow, pro-agricultural, pro-industry, etc.
How you vote affects your relations with other empires(The Pink Artisans like when you vote for peace, The Gray Industrialists like when you vote Pro-Industry) and the bills that are passed affect your empires economic system.
You can also trade with other empires. You can open trade routes which increases resources for both empires(You don't need to know that you're trading wool for fur, the player just needs to know that trade moves the supply curve to the right). You can send troops to both support the current government or troops to support any insurgencies.
When you hover your mouse over any empire it will show it's current population, it's political tendencies(pro-war), relationship with blood legion and blue army, and the strength of insurgent elements(and whether these rebels are pro-Blood Legion, pro-Blue Army, or both).
If you give too much military funding to government or insurgents or provide too much troops you can hurt your relationships with other empires.
You can also interact with the other governments via spies.
You can allocate resources to intelligence on the resource screen and then on the diplomatic screen you can determine how you spend these intelligence funds by percentages(counterintelligence, on blue, red, green gray, etc.)
You can check boxes next to the empires for what your spies do while they're there. The boxes include: Incite Rebellion, Damage Population Growth, Damage Resources, Damage Military, Damage Technology.
What needs to be done now to help prepare for this expansion pack:
-Other empires need to be able to gain and lose resources too
-It needs to be possible to add other empires