So you want to create a massively multiplayer strategy game. How do you go about designing something that will keep players coming back.
In a MMORPG, players are building up their characters. And those character die but they come back. In an MMORTS like this where city creation and its customization are what the player is worried about, then losing a city becomes a serious problem.
Therefore, the first challenge is to narrow down what exactly the player will become attached to and make sure that that attachment is not imperiled (at least not in the premium version of the game). Players will need to have tangible things to care about for the persistent world to draw players back month after month.
In the MMORPG, the character is the only thing the player really cares about. Gameplay is to make the character more powerful by acquiring new skills and items which directly impact future gameplay.
You don't want players just battling it out forever. There has to be something specific for them to care about. So what do you make them care about? A ficticious ruler? Their lands? Their people? Whatever it is, it can't be too abstract. There's nothing abstract about your Paladin in World of Wacraft getting an epic dual handed hammer of doom.
Many role playing games are already well suited for a massively multiplayer setting. Diablo practically was an MMO. But you can't just take Warcraft or Rise of Nations or whatever your favorite RTS is and make it much bigger with people fighting it out all the time. There have to be opportunities for players to savor what they're building up. And that means deciding precisely what the player is building up in that persistent world.