I have 3 suggestions so far.
co-op : two players controlling the same city to survive. nothing more to explain.
2 or 4 corner deathmatch : 2 or 4 players start in the corners of a map (probably not a random map)
and they have 3 in game months to build up their army before something starts forcing them to go to the middle to fight it out.
massive online capabilities : one very large map in wich people randomly spawn into. it would have to only load in a small area to the players or it would get laggy…or many small connected maps in which you can send raiding parties or traders to other settlements. This mode would run on a very slow speed and never stop to keep realism.
I’m just joking about the game taking forever for multiplayer, you are doing awesome on the game!