Tabletop or computer-based Narnia RPG? Tabletop RPG’s are so much more fun - in fact, I’m not 100% sure one doesn’t exist already. Heck, the local gaming store even has an Evil Dead-based RPG, so I’m sure you can find a Narnia-related one somewhere...
I’ll ask Richard, a friend of mine (I run with a strange crowd!) who develops and publishes RPG’s. Ever heard of “Bureau 13” or “Fringeworthy”? He’s the creator. He keeps up on gaming news and knows a LOT of people in the biz so I’ll check with him.
I’m betting it’s tied up in licensing. Boy, I’d buy the Sourcebook just for all the lovely detail out of the Silmarillion!
The trick would be getting the RPG to reward upright behavior, and discouraging violence while rewarding skill therein when unavoidable.
Being interested in game writing, I’m puzzling over the problem of porting books to games: the latter focus on achieving an unbroken chain of successes, whereas stories are interesting precisely because the protagonist _fails_ to some degree. Ex.: “Romeo and Juliet: The Game” would suck precisely because the end goal would have to be “and they lived happily ever after”; a tragedy is great material precisely because the main character _loses_.
Something very much like it has been out for years; it's called "DragonRaid". Your d10 rolls set up your Fruits Of The Spirit scores, and skills are tallied off of them. I found the game a little too High Church, so I wrote some mods to make it a bit more like other RPG's. You can find a good overview of both at my site...