Yeah its just crying out for multiplayer, but not just LAN. Maybe not PvP, i think that would need a fictitious balanced map (many of them really), and therefore would be a major project, but co-operative would work almost "as is" surely... i'm guessing it would be relatively simply (programming-wise) to hand over control of 2 factions to 2 seperate players (or even multiple), rather than just one faction to 1 player. then its just a case of communication and synchronisation between the players. id imagine that would work by one player hosting the game and the additional players connecting to the host. If control of a faction simply reverted back to the AI if a player leaves or DCs then that wouldnt seriously impact on the others enjoyment.
If this never makes it to Gold then it will make it into one of the future series... and i'm sure there will be many. The Roman Empire...? The Mongols...? Shogun...? American Colonisation...? So many potential historical scenarios. Would be like the "Total War" series but done properly. And your loyal fans eagerly awaiting the release of each one. But get multiplayer into them sooner rather than later please :)