Personally, I've just gotten back into EvE Online...Although it is refereed to as a SIM more than a MMORPG. It has alot of qualities some games don't offer, That is which the Player Owned Station...Most people that see this think, Well that is cool but easy right? Others would assume it's some HUGE difficult task that would require a person with no life, and 100 no life friends to accomplish. But CCP has done a GREAT deal with it....I personally just put mine online a few days ago, It's a base of a Command Structure, Being a Control tower, Then you anchor arrays, silo, storage containers and such around the control tower, And the tower then feeds fuel to itself and the other anchored structures to work...So you've gotta continue to feed the beast if you will.
My point after this wall-o-text is to do something like that...Stronghold 2 is it's own thing, IF they allowed that kinda of creation it would be AMAZING, But hard, But If they wanted to do it I'm sure they could. As a easy way though, You could have different Tiers of castles, That are bigger the higher Tier you get. Which then requires a more active roll in maintaining the structure. Instead of fuel, maintenance would work well I think, Maintaining walls, storage areas, living quarters. Just to name a few, But eh. Just a looooong idea