[Malorn]

Rather than providing higher rewards I think simply weighting the system such that it prefers to put missions against the empire that has the most territory is a natural solution.

How rewarding missions are in general could go a long way here. If missions are very beneficial then the empire's "high command" could basically work against the forces that would try to take the path of least resistance and instead encourage empires to fight each other roughly evenly or counteract the tendency to double-team.

For example the mission system could give the empire that controls the most territory an even distribution of missions to attack both enemies. Or when the empire that's not locked in the big fight gets a mission, the other two empires also get little missions that would thin out their forces a bit and attack the 3rd empire.

Lots of potential in the mission system to help correct the global situation. I'm writing this hoping that global territory and the patterns of global movement that we observed in PS1 can be taken into account for PS2. This mission system can be used to prevent blobbing into all one area and keep all the empires attacking each other rather in a way roughly consistent with both territory ownership and current population taken into account.