Soren Johnson, the lead designer of Old World, as well as Civ4, has given a talk at the GDC, which is now freely available here.
In this 1 hour presentation he talks about various game design issues, including that many decisions are mostly trial and error. He e.g. touches on that you need to know your community, otherwise you might be implementing system, which are just not desired by the community, as the MP mode without characters in Old World, or how the governance system in Civ4 was implemented.. He talks about other 4X games such as Crusader Kings or Victoria, and that murdering your family in Crusader Kings had a valid historical background.
If you want to watch his complete, very diverse talk, then click here, and discuss this talk with us here.