After giving Elite Quests a try, I think that their rewards should depend on time needed to finish them. Cause I'm getting same reward for listening to two and to five songs, and I could run Elite Quest twice, get x2 or x3 songs, and end with more experience :)
One of the points of the patience quest was that there is supposed to be a "jackpot" element to it, so you will be lucky if you get a listen 2x or unlucky if you get a listen 5x.
EDIT: But, I think giving rewards based on time required to complete, as well as difficulty, is a great idea.
That one has no level cap. I needed something to allow the higher leveled players to dump their skill points on.