I was kinda thinking I could use XML for any number of things. My first goal is to create the parser, of course. I could make a way to load custom maps from XML files, I could use XML to store configuration data for a game(i'd use a marshalled *.rxdata file, but it wouldn't be customizable outside the game), I could use XML to keep a list of add-ons and mods if i ever made a game capable of handling them... there's many possibilities.
I think I'll try making a map loader after I make the Parser, as more of a test of what I can do.
Oh, and on the subject of marshalled ruby objects, have you ever looked at the text-equivalent of one? It looks amazingly ccomplex, but if you think about the way it was saved, it's basically just "this variable is equal to this value".