Honestly, you can look to quite a few text based games for a some cues to this type of gameplay. Players are capable and interested in a more political game sometimes than a lets kill the world game. I would suggest letting each player enroll each of their characters in the campaign and it would probably work best in a Shadows ruleset or Dregs where everyone has to flag a guild at the get go. Make it so PvE doesn't permanently kill a character out of the game. Only way a character gets knocked out is through PvP. The first few rounds might be a free for all, but as guilds set up and the usual suspects get figured out you have a less likely chance of random PKs roaming around if everyone is flagging a guild, people will work extra hard to figure who they are aligned with and the end result could cause wars that wipe out entire guilds before getting close to the end. Make it a very good reward to win and you will have lots of people attempting to get to the end of the round not willing to risk getting knocked out of it.
The premise could center around resources..... the guild that mines the most resources by campaign end wins the round. The idea is to store the resources they need to take a stronghold and have storehouses to fill up with said resource. Could center around having the guilds mine and then defend the caravans that transport the resources back to the guild stronghold. Only way to know how much one guild was hoarding is to sneak in and check their storehouses, or siege them and find out. When a war finally does come around it will hold a lot of meaning, one group gets wiped out or completely killed off the winners of the war have really accomplished something. A kill on a major leader or top player will have an actual effect on the outcome. Killing the main crafters for one side will cripple a guild or alliance.
Lots of interesting scenarios could play out in a 3-4 week campaign perm death round.