Ah...what a fantastic task you have ahead of you! I've often found that while the world itself is important, the characters that populate the world are just as important. And as this is an RPG of course you'll want to spend some time on them.
I want to write a longer response to this, but I'm at work and probably should be working. I'll gather my thoughts and will probably end up responding several times.
Let me start you here, though: The Hero Myth. Read anything by Joseph Campbell. His most famous work, "The Hero With a Thousand Faces" can be found here: http://www.amazon.com/exec/obidos/tg/detail/-/0691017840/103-8297453-4595020?v=glance
For an overview of the hero myth, go here: http://www.yourheroicjourney.com/Journey.shtml
While you specifically asked about core themes and concepts, and the Hero's Journey deals with the trial of a character, I think you can still find a wealth of information here. Joseph Campbell studied the myths and stories of many cultures and put them all together to create a unifying theory of the hero that resonates on a very deep level with all of us. Sometimes the world will grow out f the story you want to tell, and sometimes it's the other way around.
Ok - back to work. I'll respond to the specific world creation question later when I don't have my head in other things. What a cool project!