At first I thought you were on board.
Then I thought you were joking and there was a game called Tacitus that already did this.
Then I figured you were actually into the idea.
Then I became unsure and googled Tacitus, finding no game by that name.
Now I'm precipitiously ambivalent. And hot. The AC is out.
EDIT: OKAY you got me with the "no rules" part. What if, like in Smash Brothers where you set up the rules for the game before you play, the players all chatted on IM and set up rules. They could make deals like, okay if you want this to be urban, you can't use this unit. So they would modify the rules so that player couldn't use that unit, but the available building blocks would be urban only. Everyone would click "sign the treaty" to seal the deal before the game started. If someone changed a parameter, everyone would unsign immediately and have to sign again after they saw the change. And if you didn't get a treaty in time, it would randomize something that put everyone on relatively equal footing.
I imagine setting up an entire historical re-enactment of the American Civil War saved as a campaign file. You'd have appropriate units constructed, you'd build all the maps, set the point-based victory conditions, etc.
You want that river twice as deep? Fine, but I want a bunch of stone walls here. Deal?