My idea is a cross between a state diagram and scripts. What you'd do is draw out the diagram (for everything that goes on in the game) with descriptions for the states and transitions in text, as well as a link to a script or to a lower sub-state diagram. If you pre built some unit scripts beforehand, with basic quest tasks/operations that are known to work correctly, you could lay out a correct RPG pretty quickly. Once that is done you would go in and add the detail, by adding to your unit scripts in an object oriented inheritance kind of way.