Similar but seperate to Alliances, players and AI should be able to form coalitions where the coalition forces all declare war against a single foe. The coalition lasts for a variable amount of turns, from 4 turns minimum(1 month) to 52 turns(1 year) maybe more or until the target force is destroyed. Assuming turns are still going to be 1week long.
This would be used when one force gains an overwhelming advantage against the other forces or against forces that are nearing a victory condition. This would make end game more challenging, but also it would give a chance for players lagging behind the AI to catch up with them. You would not need to be allied with another force to form a coalition but for the duration you would be unable to attack a fellow coalition members holdings, an option should be there to drop out of a coalition early.