For reference, Naxxramas had 30 cards, Blackrock Mountain had 31, and League of Explorers & Karazhan had 45 each.
Blackrock is the closest to the 35 cards we'll be getting in the upcoming expansion. So, I would expect 2 class cards each and 13 neutral cards. Those 4 other cards I hope go to the "Explorer" classes in a way to balance out the Legendary count since the EVIL classes got Galakrond.
They could be hero cards themselves or maybe new Legendary Spells. Either way, something to level the playing field lore wise would be fun.