If you leave out the stupid race against the clock (I have a timer-aversion in games, not sure why but they stress me out to an extreme that ruins any fun), I quite like the concept of M+. Never got to the point of modifiers i WoW.
Generally I think it would be a cool Idea but nothing that would work if you throw random people together. Hell they STILL can't handle Vishaps Ballistae and cannons, how do you expect them to deal with a pimped Garuda?
So it would be content solely for fixed groups, which would make it rather uninteresting to the masses.
As for the reward structure: yeah, if you implement scaling that intensively, character power becomes a moot point b/c content always scales up.
So what do you reward? Even more cosmetics?
We both know that content won't get accepted unless it provides player power.