Well, there are already hard and hyper modes of all dungeons coming. The only difference between that and what you are asking is it would include going through the story again. I'm not against that but I don't think that would be very popular in an MMO setting. If it were a single player title, maybe....
I kinda figured you meant that, like having the Story Line quest solo fights also more difficult. (Level 15 White Demon un-nerfed). I was just saying that in an MMO, especially this one where one character is enough for all classes, repeating story quests and such would just become tedious. Its just my opinion though, I'm sure others would love the added challenge. The only way I could see this as being something desirable is if you could farm endgame gear from it. Would it be something that would level sync you and just make it hard? So many questions and technicalities. I couldnt see this even being a whisper in the dev teams ear until many years down the road, if at all.
Not trying to burst your bubble, its why i mentioned the hard/hyper mode dungeons coming. Its about as close as you will get.
(forgive me for editing this post to respond, I hate this posting limit per day...)