If I had to design a quest I would love to have one in a solo instance (no flames please hear me out) that tests a player's abilities and understanding of what is possible with the class. One that is not so much gear based but an adaptive quest that looks at the player's general build and level and pushes the player to advance their understanding and mastery of the class. To simplify the quest design it would have multiple options to pass certain obstacles. The reward for such a quest would be simply unlock a title or some small token as a sign of recognition but would not be unbalancing so that someone was required to complete the quest in order to continue on with the game.
So if you had to come up with the premise of your ideal quest what would you like to see?
I Do like this idea. It reminds me of the Hunter quest for Lok'Delar and the Priest quest for Anathema/Benediction in wow. Those quests seemed to push you to be as creative and clever as possible to complete them, and in return you got a pretty decent(at the time) reward.
I would love epicly hard quests that pushed me to my limits as a player. It could go farther than my soul. It could look at my calling, and set up several situations and I'm required to figure out what souls would be best to do the required job. End result would be a title or something else cool.