Originality is somewhat overrated by gamers imo. You look back at the classic games, oftentimes they weren't necessarily ground breakingly original so much as they refined the genre tropes and systems or had certain gimmicks that enhanced the gameplay or just did whatever it was they did superbly well. Aside from Diablo, Blizzard has never really innovated a new game style so much as refined what had been done and added their own flair. Warcraft, Starcraft, etc were great RTS's but they were hardly the first. WoW looked at what other MMO's had done well, but also in particular what they hadn't done well, and then made a license to print money.
Hearthstone, in my limited experience with it, attempts to remove the clutter and somewhat obtuse mechanics inherent in TCGs and strip it down to essentially it's funnest components. I can't comment on their MOBA style game as I have no experience with it, but I'd assume it would be somewhat similar.
That's generally Blizzards philosophy. Take something that's been successful and strip away all the things that are a barrier to it being fun, and then enhance whatever is left.
This game looks awesome, and certainly if it delivers on the trailer it'll have more than enough gameplay mechanics and variation to separate itself from the pack.
And if critical acclaim and sales are the metric for success, Blizzard has yet to make a misstep for decades.