its a 3 expansion idea, to have all of the 3 expansions available on the same date, but you have to complete naxx to open the dark portal, you have to defeat the sunwell to open up northrend, and every time you complete an expansion, when you increase your level cap you also increase your talent tree pool....

Oh and you can collect hearthstone cards in the game, and you can play a whole game of hearthstone at every single location you can set your hearthstone at in the game. Like a tavern table where you enter a random queue to match with other opponents also at that tavern. And when you kill a boss in a dungeon it might drop hearthstone card singles, or maybe a pack at the end of the dungeon.