For save games you can't use the content pipeline because the pipeline can only be used at build time. So if you want to load a saved game - so the player can continue from where they saved, don't use the content pipeline or xnb format, just use IntermediateSerializer directly with plain xml. I don't know how easy it would be for players to load the saved game onto their pc to tweak it though..
Game hobbyist hell-bent on coding a diabolical Matrix