So far it's a forced multiplayer- and it plays out on a world map much like Mount & Blade or the Total War series. You take charge of a single faction/nation, and you organize the nation accordingly in order to rule the land. It would borrow from Mount and Blade in that it would take place in an alternate world, so as to remove any historical relevance, and it wouldn't operate in turns but instead be pushed forwards by the slow churn of time- call it a day a minute, or a day a second.
For best results, have the map auto-generate at the beginning and all civs can start from scratch.
The way that it differs, however, is the complexity. Much like some new games which involve players working together to crew a starship, this one requires teamwork in order to rule a nation. So far I've narrowed it down to 4-player co-op. One in charge of politics, one for war, one subterfuge, and one on finances.
Each of these areas would be finely balanced, infinitely complex and inextricably linked.
Take subterfuge, for instance. Agents can be hired and kept on retainer based on a budget that finance-guy gives you (already complex). However, agents must be carefully hired to prevent a fiasco (which politics guy would have to mend), or worse, double agents. They can either be hired directly in their positions or insinuated into a position after training, each with it's own risks. They then need someone to report to, and a way to. You can choose how deep the chain goes- and best of all, information doesn't get back to you until it has been moved from your imbedded agent, through the chain in that settlement, to a runner, and back to your capital. Skilled agents in cities might be able to mitigate disasters on their own initiative, but those skills can only be identified by a 'trusted' agent skilled in training and appraisal.
Add to that- different agents are needed in different areas. Palace, market, slums, army. Multiple agents in a single area help round out and confirm points of information. You have the option to allow your information gatherers to filter out information so that you don't have to churn through kitchen gossip- better hope they're skilled at that too. If you sense a net is closing in a settlement, you've got a hard choice- either pull or "silence" your agents there, leaving you blind but leaving no loose ends, or have your agents sit tight and let it close on the assumption that they'll only take a few of your agents- and possibly some agents from other factions who may have their fingers in the pot. Counterspying is necessary, and another drain on your funds.
Through this the single subterfuge player could have tens of agents in each settlement, runners constantly shifting back and forth on the map, attempting to pass the front lines between warring nations. They could even move themselves in order to get fast action on areas of importance- although that would result in a lag of information as the runners are redirected. That's why four players- a single person couldn't possibly run a country doing all four of the incredibly micro-intensive jobs at once- with the days ticking by, areas would lag behind, create ripple-disasters and ultimately cause the fall of the nation.
This is obviously a much broader idea in my mind than is possible to write down, but the idea makes me giddy. For extra realism you could even have the player characters as pieces on the board themselves- able to be promoted and demoted against their will according to the flow of the great game. If AI isn't enough of a challenge, why not have enemy nations with their own 'secret group' of four players. Still not complex enough? Why not competing groups of four Within Your Own Faction- working for themselves, their group, their political party, and your faction. If it needs to be more delicious, have no groups at all and make it open online multiplayer, with heaps of player agents, politicians, generals and scribes competing, forging transient alliances in order to net themselves more power and attempting to make their mark on the world.
Of course, I'd imagine positions could only go so low. I can't imagine the minigr me required for a player to act out being a solo agent or faceless soldier in a platoon, but it sounds boring.