I like this idea, it's like the concept that was used in Romance of the Three Kingdoms. So if any one power becomes too great, the weak can join together to fight them off.
And this is ENTIRELY different from Alliances. It would be a goal of all the weaker forces joining together to fight a strong or more overwhelming force. Thus it can also act like a balancer in the game to help prevent the super kingdom landslide, where if someone becomes powerful they just steamroll all the weaker forces.
Overall this would be a great thing to work into this game because it can give players a challenge from all sides (if they form a coalition against you, you may lose large amounts of trade). And you may be forces to fight on multiple sides instead of picking a single country to pick off one at a time.
I think it's a great idea, and it could easily prevent someone from being too powerful, but any country could rise from the ashes of the coalition to become more powerful than the others.
This would be a great logical choice that could be enabled/disabled in the game, and could tie in the the United Planets! Everyone votes for a coalition due to this one person being too powerful and evil, so they all join in to fight the same foe... (or go the route of the Romance of the Three Kingdoms, where one person initiates it and it then queries other teams or people one by one if they want to join in (making it have variable strengths by the number of people))
Just my ten cents.