It could be fun. So, so many people have asked SE for something similar to M+ in WoW, and while the dev team has been pretty clear about having no plans for such a thing, I believe they may have mentioned something about increased difficulty.

What about a timed mode? Extra chest at the end if you finish it within a specific amount of time? That could be fun. (Of course, it would be an alternate mode so that people who don't want it aren't forced to run that fast.) Just an example. There's several things that could be done to make dungeons more interesting, I think, without falling back to the M+ model or something like it.