A set of rules conducive to quick play and ease to create. Stick with one set of rules. Steal ideas from other rules!
What I am talking about is simple. Many of the modern sets of rules, even though great in their own way, would crush an Expeditionary Campaigns such as the one we are suggesting. The reason so many groups became so large is the rules themselves were so simple that they could facilitate such huge growth. Notice that there are huge societies today with huge numbers but due to the rules, mostly the publishing company releases the adventures. Is this the main factor that the huge multiple game master and huge player groups don't happen anymore?
My rules Back to the Dungeon RPG were written with the Expeditionary Campaign in mind. Check them out if you have not.
Plans for economics and politics.
Have the groups form economic and political intrigues between each other. Another group could be sabotaging the wheat and dairy production, not for war but for economic takeover of the industry and to gain power over another nation without a drop of spilled blood. Does the kingdom upstream have the right to dam up the river to flood his crops each year? Why are hunters from the next kingdom hunting in our nation's forest? If characters of are royal blood make one character in another group heir to the king IF the current character of noble blood in the other group dies.
Plans for warfare!
When the talking fails and the victims of economic schemes become tired of victimization, war occurs. Make the wars good and deadly. Make them suck. Make things precious destroyed. Many times wars in the past took place in a battlefield and there were rules of engagement. This may or not be the case.