One core concept that has been done, but never been done well, is the rune caster, most prominently on display in Weis and Hickman's Death Gate Cycle. It may just be because of inherent balance issues (i.e., Patryns and Sartans are essentially allowed to cast any spell they want, any time, infinitely), but the core concept / fluff for a character like that is fascinating, and no one has ever really captured the real flavor of a character like Haplo the Patryn.
Monte Cook's Arcana Evolved runethain is in the general vein, but severely weak compared to the book characters; the "truenaming" system in the 3.5 magic system variant splatbook (whose name eludes me) was another attempt at capturing the same type of flavor, but did it horribly, and with awful mechanics.
While I'm thinking about it, I've also never seen a good approximation of the character Garet Jax from the original Sword of Shannara. I'd imagine he'd be a weird monk/fighter hybrid, but to actually try and spec out a viable build of the character as described in the book would be a real challenge.