Mythic+ is a side game. It’s fairly popular
at which point dungeons are considered side game ?
When it’s unnecessary and the mode itself doesn’t advance the plot. It was added as an after thought to keep you active. The affixes are not canon.
I’m not saying it isn’t content. And i’m not even saying that what I said is a hundred percent true. It’s not a hundred percent false either
It’s past time to get creative with content.
With that in mind, what makes you think another area wouldn’t be just as deserted?
I’ll probably ruffle a few feathers here but I feel like saying the darkmoon faire is the same as the golden saucer is as silly as saying garrisons are housing.
Dodgy carnivals and gleaming casinos are not interchangable. 
Something like Las Vegas?