How do I make an in-game object editor that can save them as assets?

I want to make a customizable editor that I can fill with prefabs then build then into an object that can be saved/loaded for use elsewhere. Something similar to the Spore Creature creator, the vehicle editor in Banjo Kazooie Nuts and Bolts, or even the creation abilities of LittleBigPlanet. If I can just figure out how to make the framework I can make different versions for each object type and build out my game world that way.