I think the Demon Hunter cards are gonna be from some outside force, like Death Knight cards are via The Lich King... not sure though. It would be ground breaking... but also would pose a lot of questions IF it was true that it would be added to main pantheon of classes.
Firstly: Old expacs.
Blizzard would have to either: A) Add only basic/classic and this expacs cards to the class. B) Add cards to standard only or C) Create cards to all expacs there were in Hearthstone ever.
Option A is just lazy. Option B is the most optimal BUT, and that's a big but, the arena. If only the standard expacs would get DH cards then the arena buckets would include way less class cards vs neutral ones, since arena usually include both standard and wild expacs in rotation. Option C would be the best, but also the most exhausting for the Blizz to take. I would be glad since I'm a big wild fan and I watch wild youtubers way too much, but I really think the B would happen at the best case scenario.
Secondly: Blizzard stated once they had no intentions on adding the new classes as a pickable option, that's why Death Knight's cards ended up as uncollectible cards from Arthas and his dog.
All in all... the best bet I have is that these will just be another DK uncollectible scenario, since it would require too much work to create new UI and cards for all the standard expacs (or all if they would fish for bigger fish) AND do expac for all other classes in the same time, we're talking like at least 150% of usual HS expac.
Veteran of using wild decks via deck helper only to have lil challenge and because standard is boring, which thinks that RNG is best part of Hearthstone (We all love Unstable Portal and Spellslinger or when Mad Bomber destroys our Piloted Shredder from which you get Doomsayer which destroys entire board). Seeking players which would be want to play random decks on EU. only what you need to have is accurate Hearthpwn collection, how to I can tell after adding me.