The multiplayer campaign is a really great feature that needs to be kept in Rome 2. It is my contention that it is easily possible to improve it further by allowing 4 players to take part in a multiplayer campaign.
As I see it, the biggest problem with increasing the number of players is the time spent in battles. As of now, it is easily possible to limit the turn length to 2 minutes per player, which makes it quite possible to enjoy a full 4-player campaign. But no one wants to spend 20 to 40 minutes waiting for 2 players (or maybe only just one) to complete a battle. As auto-resolving every battle doesn't seem very interesting to me (I truly believe that it would kill the essence of the game itself), I think that we need to come up with something. That's where my idea appears: we should make a 2 vs 2 multiplayer campaign.
-During the other players' turn, each player should be able to manage not only construction but also to use diplomacy and to give movement orders and manage taxes. When it is the player's turn, your ordered movements should take place and you will probably not take too long to end your turn. This combined with a timer for the time length would ensure that the game runs quite smoothly.
-During battles, this is the real moment when the 2 vs 2 alliances are needed. Every time a battle occurs, it will involve the 4 players. The player attacking or defending will of course have control of his own troops and he will be able to give part of his troops to his ally, just like in the current coop multiplayer campaign. The IA controled troops will be given to both their enemies, like in Napoleon Total War's multilayer campaigns.
-There could be different sets of victory conditions: either there could be a prestige victory condition (based on marvels and important buildings for instance), a domination victory condition or even a diplomatic victory condition (if you have enough support among the IA-based factions). The first team achiving its victory condition would win (obviously).
I think this would allow for a challenging and intersting new multiplayer game-mode.