Well, I'd concentrate on breaking items into types with attendant resources and then work on the rules for how these resources can be affected. A good place to start might be in terms of proprties of substances and the forces or items that can affect them.

For instance, maybe you have wood as a property. If you use hit points, then maybe you set up some sort of rule for how many HP wood has, maybe even how much HP per pound / kilogram.

Then maybe you describe the forces/items: electricity, fire, acid, etc. and how they affect wood, in points. You can later add elements like buoyancy or buttressing (say for a barricade), but you should keep it focused strictly on what the game can do.

One caveat is that the more detailed you make the properties, the more people are going to expect all properties to be detailed. This can be a real challenge unless you can artificially limit the environment (say, a biome or underground cave).

An idea I had quite some time ago, was more or less a persistant world.
A single player MMORPG if you will.

Basicly the entitys (NPC's) would go about their normal tasks. Building stuff, Working in buildings, Commiting crimes, Arresting those who had broken the law.

A world that would work even if the player just sat there watching it all go by. The player could persue the story, or just explore the world.

Of course if the player didn't persue the story in time, then something bad may just happen to this world. But that was happening latter.

I still plan to try this at some point, but It's no small task.

Armand.