The only difficulty setting that would get interesting to add to the game is a Hard mode for solo instances.

I don't want them because I hate them all, but for those who love them it would be a fun addition.

If we're talking about CONTENT difficulty... Savage 4-man dungeons anyone? I have little hopes Criterion Dungeons would fill the midcore itch but it would be a start. Just difficult 4 man content at the equivilent of EX or Savage or ARR Hard Dungeons would be welcome.