Oh, I totally see your point on "commonly acceptable prompts," but if you want to pick a "grand prize winner," and then do some ranked choice voting for additional prompts or whatever, you could? The fun is the point of a prompt showdown! The campaigning for votes! The sheer OTTness of it all.

So I should say that even for CNTW, it is good tagging practice to also warn? But definitely if you have some fraught topics, you should also choose CNTW! I will use myself as an example, my latest fic is definitely a CNTW, I also have Dead Dove Do Not Eat, Graphic Violence, Consensual But Not Safe or Sane, and many other, worse tags on it, because it's a real humdinger of a dark fic. (I won't link it. I'm sure you could find it if you wanted.)