It took the total war guys a super budget and 15 years of development to make a multiplayer campaign, and even then you can only have two players at the same time. While I love it, especially in Shogun 2, I think it is a major coding issue for the developers. For this game, it would be a pretty intense issue that would change the basic nature of the campaign. The most logical implementation I could see is if you had drop in matches like Shogun, where you can get internet players to join your battles as enemies. For a whole campaign, while awesome, seems far away.