Well, I was disappointed with the Death Note fan fic, because it doesn't seem to have added value beyond the original story. And I agree that exploring the supernatural should be a high priority for a rational person, once the supernatural is experimentally proven. Would it be so difficult to ask Ryuk whether there are additional magical items that could also be abused? I guess Ryuk would use an excuse of having "rules" against that, but at least it's worth trying.

Having a rational superhero is a necessary condition for a rationalist story, not a sufficient condition. Ender's Game could be a rationalist literature if it explained Ender's reasoning better, and if Ender strategically tried to improve his understanding of the world. Okay, another necessary condition is not just that the superhero is super smart, but also that the super smartness is at least partially a result of a good strategy, which is shown to the reader.