I've been writing a set of rules for the Warring States period, and Arjun and I ran a play-test today.
I'll post the rules up when they're completed, but in a nutshell, it's a grid based system, with a focus on replicating the historical features of the period as represented in The Art of War - namely a differentiation between direct and indirect attacks (or orthodox and unorthodox, or ordinary and extraordinary, depending on the translation), a "rock paper scissors" mechanic between different troop types, and an over-arching army morale system based on a "Ying and Yang" mechanic.
... all of which makes no sense whatsoever unless you read the rules, so I'll shut up about the details until the rules are ready to post. The game itself worked quite well in highlighting various problems with the rules, and I'll be making improvements for the next game.
It was good to finally get the Warring States figures out for a game. Looks like we've got a new campaign in the works ... :)