The only real snag I see with it isn't the player element so much as the fact that this would require far more testing than I'm convinced devs actually do for what content we already have.
On the other hand though, I suspect the workaround would be the S3 treatment of setting particular numbers and setting ranks based off of numbers revolving around various factors such as speed of completion or number of deaths, and much like how you could calculate the time to down particular S3 dummies or the percentage leftover when the timer runs out to determine a rough estimate of how much dps you're doing, you could use the numbers gained from those factors to generate a "rank".
Two questions regarding this though: 1) would devs actually want to dedicate the assets necessary for a system like given that likely none of it would involve any amount of actual player testing, and 2) what would be deemed acceptable testing values (i.e. number of runs, number of groups testing) to determine the parameters for each rank of each dungeon?
