I don't think anyone is under any surprise that demon hunter is probably going to be the most played class in the game come next expansion. The already dwindling rogue population will no doubt take a massive blow. Monk's are cool and all but nobody really envisioned them or ever thought "hey I want to play this in the future maybe."
Demon Hunter however, that's been wanted in the game since forever.
This is also an issue for raiding. ANOTHER tank is probably not something that was needed, let alone another melee. Black hand mythic required 3 melee. Raiding in general has always been tight for melee and generally melee unfriendly. This is only going to make spots harder to get now with this new class.
I know it's a lot to speculate right now but I can't be the only one out there worried about class population and distribution next expansion.