I am against Mythic+ dungeons not because I don't think it would work but because I would rather see other types of difficult content besides dungeons. PoD's lower floors are a great example of a difficult challenge that can be rewarding (lots of folks haven't done that). The problem is people want hard content without increased time investment. It takes a while to clear down PoD, people get bored and want out, difficulty aside.

Someone earlier mentioned a 'Boss Rush'. That's something that could be challenging quick and fun.