The problem is if the content is required for the MSQ. Anything that is required for the MSQ needs to be easy because some of the people who play only for the story are not very good and would be unable to progress if it was difficult. They would quit the game instead. Now that we are getting trusts in these dungeons, they are being nerfed a bit more so an AI can do them.
We used to have optional "hard mode" dungeons. There were apparently some very hard ones in ARR, but I didn't know anyone who regarded them as being hard when I started doing them.
A dungeon with multiple difficulty levels would be ideal, so we should wait and see what criterion dungeons are like.