The second and as of writing, the second expansion pack to Shogun 2 and wow is it worth the money. First of all you get 6 factions in the campaign mode which is an extra 60+ hours of gameplay alone and for a mere £2 extra (basically less than the price of a cup of coffee), you get an extra clan that will add at least 10+ more hours to your gameplay.
The game engine is now better optimised and much more stable and also runs the game more smoothly on mid to low range machines capable of playing it. Anyone who was having trouble running the game beforehand should give it a go.
Multiplayer has basically gotten a huge expansion too given that the game's added well over 60 new land and naval units plus a 19th Century avatar skill tree. The combination of 12th, 17th and 19th century units also adds a huge variety of tactice and batttle/play styles so you should easily find one that suits you.