A philosopher as the main character of a fantasy world -- now there would be a REAL challenge, whether they're male or female. (I think. Perhaps I'm just unimaginative.)
I'd like to echo darkredd's request -- would you be willing to do one on antiheroes? (As opposed to just villains, pure and simple.)
I think it would be. Most of the time, there isn't true philosophical debate in a fantasy world. Instead, there's just the history and truth that Everyone Knows, or at least that the educated people know. (The closest I've seen this come is when there are two sides, and of course one side is wrong and one right).
As soon as I can clarify my thought on antiheroes, I'll try to do a rant on them.
In terms of fantasy books where you have two sides fighting, and neither one is wrong or right, I'd like to suggest Holly Lisle's 'Talyn', which came out in 2006. It starts off with your two typical fantasy countries locked into a 300 year old war, and then proceeds to have a major character from each land provide insight into the story. Both characters are convinced they are right, that their land is the more just and equal, and... you get the point. Later there's the introduction of a third side, and a critique of colonialism and interference that's like a slap in the face, but I still value the unique qualities of the book. My best friend hated the second half with the evils of colonialism stuff, so it's not for everyone, but I was fascinated by the conflict and later debates between the two main characters about why each of their countries should be ascendant over the other.